Compositions and methods for producing microfibrillated cellulose with increased tensile properties

ABSTRACT

Methods for preparing aqueous suspensions comprising microfibrillated cellulose, and optionally inorganic particulate material, with increased tensile properties, and methods for selecting a fibrous substrate comprising cellulose for preparation of microfibrillated cellulose having increased tensile properties.

FIELD OF INVENTION

Methods and compositions for preparing microfibrillated cellulose with increased tensile properties from fibrous substrates comprising cellulose feed stocks selected on the basis of zero-span tensile index and hemicellulose content measurements of the feed stock.

BACKGROUND OF THE INVENTION

Various methods of producing microfibrillated cellulose (“MFC”) are known in the art. Certain methods and compositions comprising microfibrillated cellulose produced by grinding procedures are described in WO-A-2010/131016. Husband, J. C., Svending, P., Skuse, D. R., Motsi, T., Likitalo, M., Coles, A., FiberLean Technologies Ltd., 2015, “Paper filler composition,” PCT International Application No. WO-A-2010/131016. Paper products comprising such microfibrillated cellulose have been shown to exhibit excellent paper properties, such as paper burst and tensile strength. The methods described in WO-A-2010/131016 also enable the production of microfibrillated cellulose economically.

In an embodiment of the process set forth in WO-A-2010/131016, the contents of which is hereby incorporated by reference in its entirety, the process utilizes mechanical disintegration of cellulose fibres to produce microfibrillated cellulose (“MFC”) cost-effectively and at large scale, without requiring cellulose pre-treatment. An embodiment of the method uses stirred media detritor grinding technology, which disintegrates fibres into MFC by agitating grinding media beads. In this process, a mineral such as calcium carbonate or kaolin is added as a grinding aid, greatly reducing the energy required. Husband, J. C., Svending, P., Skuse, D. R., Motsi, T., Likitalo, M., Coles, A., FiberLean Technologies Ltd., 2015, “Paper filler composition,” U.S. Pat. No. 9,127,405B2.

A stirred media mill consists of a rotating impeller that transfers kinetic energy to small grinding media beads, which grind down the charge via a combination of shear, compressive, and impact forces. A variety of grinding apparatus may be used to produce MFC by the disclosed methods herein, including, for example, a tower mill, a screened grinding mill, or a stirred media detritor.

MFC has been made from numerous sources including hardwoods (e.g. birch, eucalyptus, and acacia), softwoods (various species of pines and firs) and non-wood sources (e.g. cotton, abaca, flax, bamboo, and sugar beet. Oladi, R., Oksman, K., Dufresne, A., Hamzeh, Y., 2015, “Different preparation methods and properties of nanostructures cellulose from various natural resources and residues: a review,” Cellulose, 22: 935-969.

The fibre source can have a strong influence on MFC quality. For example, it has been found that abaca and sisal fibres produced finer MFC than flax and hemp. Alila. S., Besbes, I., Vilar, M. R., Mutjé, P., Boufi, S., 2013, “Non-woody plants as raw materials for production of microfibrillated cellulose (MFC): a comparative study,” Industrial Crops and Products, 41:250-259. Others have attempted to investigate the influence of fibre species on MFC quality, but these studies are limited to comparisons of a small number of species at a time, making the drawing of conclusions applicable to most fibre sources problematic. Chaker, A., Alila, S., Mutje, P., Vilar, M. R., Boufi, S., 2013, “Key role of the hemicellulose content and the cell morphology on the nanofibrillation effectiveness of cellulose pulps,” Cellulose, 20:2863-2875; and Desmaisons, J., Boutonnet, E., Rueff, M., Dufresne, A., Bras, J., 2017, “A new quality index for benchmarking of different cellulose nanofibrils,” Carbohydrate Polymers, 174: 318-329.

Fibres differ from each other in many ways, so it cannot be taken for granted that differences in MFC quality between two fibre sources are due to a difference in one parameter rather than another.

Despite the benefits seen in WO-A-2010/131016, there is ongoing need to further improve the economics of producing microfibrillated cellulose on an industrial scale, and to develop new processes for producing microfibrillated cellulose. It would also be desirable to be able to further develop or enhance one or more properties of microfibrillated cellulose and products comprising microfibrillated cellulose and to identify cellulose-containing fibrous feed stocks suitable for preparing MFC with increased tensile properties.

SUMMARY OF THE INVENTION

In accordance with the description, figures, examples and claims of the present specification, the inventors have discovered that high hemicellulose content fibres and a high fibre zero-span tensile index (indicating few fibril flaws in the fibre leads to the production of strong MFC). Further work has demonstrated that this influences the optimum processing conditions during production. For example, the grinding solids content affects the intensity of the process, and it has been found that fibres that produce high quality MFC benefit greatly when the grinding solids content is relatively low, whereas fibres that produce poor quality MFC show little benefit. This is believed to be due to the less intense conditions better preserving the long, thin fibrils intrinsic to fibres that produce high quality MFC.

Hemicellulose is an amorphous polysaccharide that forms a layer on microfibril surfaces, separating neighbouring microfibrils. This is expected to provide a preferred plane of breakage along the microfibril direction. It has been found in the literature that this aids microfibrillation and results in the liberation of finer microfibrils. Hemicellulose content and cell morphology play an important role in the effectiveness of nanofibrillation of cellulose pulps. Chaker, A., 2013. The inventors have discovered that the hemicellulose content of numerous cellulose fibre species positively correlates with the tensile index of the microfibrillated cellulose produced from such fibrous substrates comprising cellulose.

Specifically, the zero-span tensile index of cellulose feed fibres has been found to correlate with the length-weighted mean fibre length of MFC (defined as the largest dimension of the MFC particle as measured by a Valmet FS5 Fibre Image Analyser) produced from such cellulose fibre feeds. Zero-span tensile index is a measurement of the resistance of individual fibres to breaking across the cross-sections of the fibres. This measurement can therefore be considered an indication of the frequency of flaws in the cellulose fibre structure. By using the mathematical product of the hemicellulose content and the fibre zero-span tensile index, a reasonably good prediction of the peak MFC tensile index can be made without requiring the actual production of the MFC from such fibrous substrates comprising cellulose. This is illustrated in FIG. 1. The mathematical relationship of hemicellulose content and zero-span tensile index of the fibrous substrate comprising cellulose can be used to identify preferred cellulose fibre sources and to select cellulose-containing fibres that are expected to produce MFC with desirable tensile strength properties.

High hemicellulose content cellulose fibres with few flaws in their fibril structures (inferred by the fibre zero-span tensile strength) have been found to lead to strong MFC fibrils. Moreover, these properties have been shown to yield optimum processing conditions during production. By identifying geometric properties of the MFC measured using a fibre image analyser, it was found that the MFC fibre length greatly improves the correlation with tensile index, when multiplied by the hemicellulose content. This relation can be rationalised to fit the Page Equation, which is a theoretical model for the prediction of paper tensile index. Page, D., 1969, “A theory for the tensile strength of paper,” Tappi Journal, 52(4): 674-681.

The Page Equation is stated below as

$\begin{matrix} {\frac{1}{T} = {\frac{9}{8Z} + \frac{12A\rho}{\tau_{B}P{L({RBA})}}}} & {{Equation}\mspace{14mu}\lbrack 1\rbrack} \end{matrix}$

where T is the sheet tensile index (Nm/g), Z is the zero-span tensile index (Nm/g), A is the fibre cross-sectional area (m²), P is the fibre cross-section perimeter (m), ρ is the fibre density (kg/m³), L is the fibre length (m), τ_(B) is the shear bond strength per unit area (Pa), and RBA is the relative bonded area.

Zero-span tensile index is a measure of individual fibre strength. RBA is a measure of the fraction of the fibre surface area that is used for inter-fibre bonding. The first term on the right-hand side of Equation [1] represents the weakness of the individual fibres, whereas the second term represents the weakness of the bonds between fibres. Usually, a sheet of paper fails due to bonds breaking rather than the fibres breaking, so the second term is limiting. Adding MFC to a fibre furnish greatly increases relative bonded area and so tensile index tends to improve considerably. Lindstrom T., Fellers, C., Ankerfors, M., Nordmark, G. G., 2016, “On the nature of joint strength of paper—effect of dry strength agents—Revisiting the Page equation, Nordic Pulp & Paper Research Journal, 31(3): 459-4680.

Microfibrillated cellulose produced in accordance with the invention was used in a composite with calcium carbonate mineral to form nanopaper sheets, which were tested to assess the tensile index. The hemicellulose content of the fibres prior to MFC production was measured and was found to correlate with the tensile index of the respective MFC produced. Additionally, SEM images showed that higher hemicellulose fibres result in the liberation of microfibrils with finer widths, which is consistent with what others have reported using alternative MFC production methods. See, e.g., FIG. 9A and 9B, which depict the difference in appearance of a high hemicellulose content birch fibre (FIG. 9A) which has been microfibrillated at 3,000 kWh/t versus a cotton fibre having little to no hemicellulose (FIG. 9B) also fibrillated at 3,000 kWh/t.

Various geometric parameters of the fibres such as length and width were measured with a fibre analyser and shown not to correlate with the MFC tensile index. However, measurements of the apparent length of the MFC product particles, likely in the form of fibril aggregates (MFC “fibre length”), were recorded with this equipment, and multiplying this fibre length by the hemicellulose content resulted in a parameter that correlated strongly with MFC tensile index. While measured MFC length greatly improves the correlation with tensile index, unfortunately, this measurement defeats the purpose of being able to predict which cellulose feed stocks will yield MFC with desirable tensile properties, since the MFC must still be produced to be able to determine the MFC fibre length. Nevertheless, it is conceptually useful to demonstrate that the length of the MFC particles is important. Furthermore, this implies that a Page-equation-like model is a good framework to use for a predictive model in selecting suitable cellulose feed stocks.

The Page Equation was applied and modified, with some of the parameters in the bonding term being substituted with hemicellulose content and MFC length. It was found that the addition of a constant σ₀ to represent the residual strength in the absence of hemicellulose was required for the model to fit the data. This constant will differ depending on grinding conditions, e.g. energy input, grinding solids, and energy intensity/impeller speed. In the examples shown the constant was 4.1 Nm/g.

Using MFC length as L in the Page Equation [1] and hemicellulose content as RBA in the Page Equation, it was possible to plot predicted tensile index versus measured tensile index of MFC prepared from a multiplicity of cellulose feedstocks. FIG. 11 shows good correlation of the predicted versus measured tensile indices for a wide variety of cellulose fibres. These were Nordic Pine, Black Spruce, Radiata Pine, Southern Pine, Enzyme-Treated Nordic Pine, Douglas Fir, Dissolving Pulp, Birch #1, Birch #2, Eucalyptus, Acacia, Mixed European Hardwood, Mixed Thai Hardwood, Tissue Dust, Cotton Linters, Jeans, Abaca, Sisal, Bagasse, Kenaf, Miscanthus, Sorghum, Giant Reed and Flax.

An empirical equation was devised to predict tensile index by combining hemicellulose contents and measured MFC fibre lengths. Thus, the Page Equation was modified by:

T=1.3 (H×L)+4.1

T=tensile index (Nm/g)

H=hemicellulose content (mass fraction)

L=“length” of MFC particles at optimum energy input (mm)

Combining the effect of hemicellulose and MFC fibre length improved the fit greatly.

Furthermore, zero-span tensile index of the fibres, which is a measurement of the quality of the fibre cross-sectional area and inversely related to the number of flaws present, correlates with the length of MFC fibrils produced from a given cellulose feed stock.

Zero-span tensile index of the initial fibres was used as a proxy for the MFC length, since it appears that the frequency of flaws in the fibril structures that is represented by the zero-span tensile index, results in shorter fibre lengths when MFC is produced. This proxy results in a weaker fit, but still a substantial improvement over hemicellulose content alone in terms of predictive value with regard to the resultant tensile index of the MFC produced from a given cellulose feed stock.

The inventors herein have, thus, shown that measurements of the hemicellulose content and zero-span tensile index of pulp fibres can be used to accurately predict the resultant MFC tensile index produced in accordance with the processes described herein. Accordingly, the present disclosure provides a facile method to aid in the selection of cellulose fibre sources for use as a feedstock for the production of microfibrillated cellulose.

Using fibre zero-span strength as a proxy for MFC fibre length results in the following relation:

T=0.0021(H×Z)+4.2

T=tensile index (Nm/g)

H=hemicellulose content (mass fraction)

Z=zero-span tensile index (Nm/g)

The foregoing allows a reasonable prediction of MFC tensile index to be made based on intrinsic fibre properties that does not require a sample of MFC to be actually produced first.

The foregoing is depicted in FIG. 12, which presents a plot of predicted tensile index versus measured tensile index for 24 different cellulose feed stocks, when zero-span tensile index (Nm/g) is multiplied with hemicellulose content of the cellulose feed stock. Again, a reasonable correlation is found among predicted and measured values, thereby enabling one to predict the desired tensile index of MFC produced from a multiplicity of feed stocks.

Using the hemicellulose content and the zero-span tensile index of the fibres, a parameter could be produced that correlates with the MFC tensile index, and therefore MFC quality can be predicted from measurements of the feed properties.

Hemicellulose

Most raw plant materials from which cellulose fibres are extracted also contain high concentrations of hemicellulose. Though pulping and bleaching removes much of the hemicellulose, there is still typically a residual fraction within the fibre cell wall, with the amount dependent on fibre species and pulping conditions.

Hemicellulose is a broad term for a wide variety of polysaccharides with differing monomer sugars, functional groups, and degrees of branching. For woods and many non-woods, there are two important families; xylans and glucomannans. Xylans are found in the vast majority of plants, and account for almost all the hemicellulose in hardwoods, whereas glucomannans are found in large quantities in softwoods (in comparable amounts to xylans). Ebringerová, A., 2006, “Structural Diversity and Application Potential of Hemicelluloses,” Macromol. Symp., 232: 1-12.

Compared to cellulose, hemicellulose is always amorphous, whereas cellulose is partly crystalline, and hemicellulose molecules have relatively short chain lengths of 70-200 units compared to 300-1700 units typical for cellulose. Fengel, D., Wegener, G., 1983, “Wood-chemistry, ultrastructure, reactions, De Gruyter; Klemm, D., Heublein, B., Fink, H. P., Bohn, A., 2005, “Cellulose: Fascinating Biopolymer and Sustainable Raw Material” Angew. Chem. Int. Ed., 44:.3358-3393. See FIG. 8 for an illustration of the structural relationship between crystalline cellulose, paracrystalline cellulose and hemicellulose. Within a fibre cell wall, hemicellulose closely associates with the cellulose microfibril surface, forming a layer separating neighbouring microfibrils. NMR spectroscopy indicates that both xylan and glucomannan do this, and are comparable in function. Liitä, T., Maunu, S. L., Hortling, B., Tamminen, T., Pekkala, O., Varhimo, A., 2003, “Cellulose crystallinity and ordering of hemicelluloses in pine and birch pulps as revealed by solid-state NMR spectroscopic methods,” Cellulose, 10:307-316. Hemicellulose has a branched, amorphous structure, and readily swells in water, as shown by work investigating the change in zeta potential during this process Uetani, K., Yano, H., 2012, “Zeta Potential Time Dependence Reveals the Swelling Dynamics of Wood Cellulose Nanofibrils,” Langmuir, 28: 818-827. This hydrophilicity also aids in the plasticity of the fibre to deformation, which would be expected to make disintegration into MFC easier. Bolam, F. M., 1965, “Stuff Preparation for Paper and Paperboard Making: Monographs on paperboard and papermaking,” Pergamon.

NMR studies by several authors using fibres that have undergone different pulping conditions has shown that reducing the hemicellulose content appears to increase the fibril aggregate dimension size appreciably. Hult, E. -L., Larsson, P. T., Iversen T., 2001, “Cellulose fibril aggregation—n inherent property of kraft pulps,” Polymer, 42: 3309-3314; Virtanen, T., Maunu, S. L., Tamminen, T., Hording, B., Liitiä, T., 2008, “Changes in fiber ultrastructure during various kraft pulping conditions evaluated by 13C CPMAS NMR spectroscopy,” Carbohydrate Polymers, 73:156-163; and Duchesne, I., Hult, E. L., Molin, U., Daniel, G., Iversen, T., Lennholm, H., 2001, “The influence of hemicellulose on fibril aggregation of kraft pulp fibres as revealed by FE-SEM and CP/MAS 13C-NMR,” Cellulose, 8:103-111. This supports the findings that hemicellulose inhibits the spontaneous coalescence of neighbouring microfibrils.

It is understood in the prior art that hemicellulose content impacts papermaking. If hemicellulose is removed prior to refining, tensile strength of the fibres may be reduced. Bolam, F. M., 1965. Adsorbing hemicellulose onto fibres prior to refining has been found to improve sheet tensile strength, primarily by reducing the ‘kink’ deformations induced in the fibres. Mäkelä, P., Bergnor, E., Wallbäcks, L., Öhman, F., 2010, “Sorption of birch xylan to softwood kraft pulps and its influence on the tensile properties of previously-dried papers under different papermaking conditions,” Innventia Report No. 121 2nd Version.

We postulated that higher hemicellulose content would lead to high quality MFC. It has been shown in the literature that drying a pulp after removing hemicellulose by alkali treating results in irreversible microfibril aggregation, inhibiting fibrillation compared to an untreated pulp. Iwamoto, S., Abe, K., Yano, H., 2008, “The Effect of Hemicelluloses on Wood Pulp Nanofibrillation and Nanofiber Network Characteristics,” Biomacromolecules, 9:1022-1026.

Numerous authors have found that hemicellulose content coincides with a high microfibril yield and better individualisation. This appears true whether comparing fibres from different plant species or from the same plant species but with different pulping conditions. Alila, et al., 2013; Desmaisons, J. et al., 2017; and Chaker, A. et al., 2013; Rahman, S., Petroudy, D., Ghasemian, A., Resalati, H., Syverud, K., Chinga-Carrasco, G., 2015, “The effect of xylan on the fibrillation efficiency of DED bleached soda bagasse pulp and on nanopaper characteristics,” Cellulose, 22: 385-395; and Spence, K. L., Venditti, R. A., Habibi, Y., Rojas, O. J., Pawlak, J. J., 2010, “The effect of chemical composition on microfibrillar cellulose films from wood pulps: Mechanical processing and physical properties,” Bioresource Technology, 101: 5961-5968.

Two important mechanisms are thought to explain this. First, is that the presence of surface hemicellulose itself improves inter-fibre bonding (or inter-fibril bonding in the case of MFC), since amorphous hemicellulose chains extend out from the microfibrils when immersed in water, and form bridges between neighbouring microfibrils when dried. Bolam, F. M., 1965. Disintegrating a high hemicellulose pulp into MFC liberates surface area coated in more hemicellulose, enhancing this strengthening effect compared to a low hemicellulose pulp. When hemicellulose is removed from nanocellulose with xylanase enzymes, this results in poorer tensile properties, even with similar nanocellulose geometry, clearly demonstrating this effect. Arola, S., et al., 2013.

Second, in addition to the foregoing effect, a high hemicellulose pulp produces finer microfibrils with better individualisation, as microscopy images in various studies have demonstrated. Alila et al. (2013); Iwamoto et al. (2008); and Chaker et al. (2013). Given similar microfibril lengths, this increases particle aspect ratio, improving tensile strength. Hemicellulose forms an amorphous layer between microfibrils that readily swells in water, and so this would be expected to provide a preferred plane of breakage parallel to the microfibril lengths, thereby facilitating the production of finer microfibrils. Additionally, xylan develops a surface charge due to carboxyl group dissociation under typical processing conditions, causing mutual microfibril repulsion, enhancing this effect to some degree. Due to its expected influence on MFC geometry and bonding, the hemicellulose content was investigated for all fibre species, as set forth herein.

Zero-Span Tensile Index

We have demonstrated in this specification that fibre lengths of MFC correlate with a high MFC tensile index. Thus, it is desirable to be able to predict the resultant MFC fibre length from intrinsic fibre properties. All other things being equal, long fibrils within the fibre structure will lead to long liberated fibrils. Also, fibrils with few defects reduce the degree of fibril length degradation during processing. Intrinsically long fibrils means fewer discontinuities at fibril endpoints. Undamaged fibrils means fewer microscopic weak points in the fibre. Both of these properties can be seen to influence the “quality” of the fibre cross-sectional area, i.e. having long, undamaged fibrils results in the cross-sectional area having few flaws.

We postulated that a measurement that could assess the specific strength of the fibre cross-sectional area could therefore be useful for indicating the frequency of fibril flaws and intrinsic fibril length; and is therefore expected to correlate with long fibril lengths of the MFC product. The zero-span tensile index of the fibre sheet prior to MFC production has been found to be such a measurement.

In the zero-span tensile test, the two clamps of the tester are essentially touching (within microns of each other), forcing the vast majority of the fibres between the clamps to be held by both clamps at once, since the separation distance between clamps is a small fraction of typical fibre lengths. When the sample is broken under tensile stress, these clamped fibres will fail, rather than the bonds between fibres as with conventional tensile testing. Since the zero-span tensile test is normalised by weight, the thickness of the fibre cell wall and fibre diameter are accounted for.

The use of zero-span tensile index as a measurement of fibre damage is supported in the prior art. Zeng, X., Retulainen, E., Heinemann, S., Fu, S., 2012, “Fibre deformations induced by different mechanical treatments and their effect on zero-span strength,” Nordic Pulp and Paper Research Journal, 27(2): 335-342. Zero-span tensile index has been shown to be inversely proportional to the frequency of fibre kinks induced (i.e. sharp bends in the fibre) by homogenization, which decreased fibre length probably due to non-uniform load distributions across the cross-section. Joutsimo, O., Wathen, R., Tamminenm T., 2005, “Effects of fiber deformations on pulp sheet properties and fiber strength,” Paperi Ja Puu/Paper and Timber, 87(6).

Further studies have shown that fibril and microfibril-scale damage is also important. Fibres treated with acid caused localised damage to microfibrils that substantially reduced zero-span tensile index. Further, zero-span strength decreased in damaged fibrils homogeneously throughout the fibre due to thermal ageing degradation. Nevell, T. P., Nugawela, D., 1987, “Effect of Treatment with Very Dilute Acids on the Wet Tensile Strength and Chemical Properties of Paper,” Carbohydrate Polymers, 7:169-181; and Wathen, R., 2006, “Studies on fiber strength and its effect on paper properties,” PhD Thesis, King's College London. ISSN 1457-6252.

A first aspect of the present invention is a method for preparing an aqueous suspension comprising microfibrillated cellulose (MFC) with increased tensile properties, and further comprising inorganic particulate material, the method comprising the steps of:

-   -   (i) providing a multiplicity of fibrous substrates comprising         cellulose;     -   (ii) determining the zero-span tensile index in Nm/g and         hemicellulose content of the fibrous substrates comprising         cellulose;     -   (iii) predicting the MFC tensile index in Nm/g from the product         of the hemicellulose content and fibre zero-span tensile index         of the fibrous substrates comprising cellulose;     -   (iv) selecting the fibrous substrates comprising cellulose         having a desired MFC tensile index; and     -   (v) microfibrillating the fibrous substrates comprising         cellulose in an aqueous environment by grinding in the presence         of a grinding medium, wherein the grinding is carried out in the         presence of grindable inorganic particulate material

A second aspect of the present invention is a method for preparing an aqueous suspension comprising microfibrillated cellulose (MFC) with increased tensile properties, the method comprising the steps of:

-   -   (i) providing a multiplicity of fibrous substrates comprising         cellulose;     -   (ii) determining the zero-span tensile index in Nm/g and         hemicellulose content of the fibrous substrates comprising         cellulose;     -   (iii) predicting the MFC tensile index in Nm/g from the product         of the hemicellulose content and fibre zero-span tensile index         of the fibrous substrates comprising cellulose;     -   (iv) selecting the fibrous substrates comprising cellulose         having a desired MFC tensile index; and     -   (v) microfibrillating the fibrous substrates comprising         cellulose in an aqueous environment by grinding in the presence         of a grinding medium, wherein the grinding is carried out in the         absence of grindable inorganic particulate material

A third aspect of the present invention is a method of selecting a fibrous substrate comprising cellulose for preparation of microfibrillated cellulose (MFC) having increased tensile properties, the method comprising the steps of:

-   -   (i) providing a multiplicity of fibrous substrates comprising         cellulose;     -   (ii) determining the zero-span tensile index in Nm/g and         hemicellulose content of the fibrous substrates comprising         cellulose;     -   (iii) predicting the MFC tensile index in Nm/g from the product         of the hemicellulose content and fibre zero-span tensile index         of the fibrous substrates comprising cellulose; and     -   (iv) selecting the fibrous substrates comprising cellulose         having a desired MFC tensile index

In an embodiment of the first, second and third aspects, the MFC tensile index is calculated using the equation T=B₂ZH+σ₀, wherein Z represents the zero-span tensile index of the fibre in Nm/g, H represents the hemicellulose content as a mass fraction, B₂ is a proportionality coefficient, and σ₀ a value of 4.1 Nm/g.

In another embodiment of the first, second and third aspects, the fibrous substrate comprising cellulose is selected from the group consisting of Nordic Pine, Black Spruce, Radiata Pine, Southern Pine, Enzyme-Treated Nordic Pine, Douglas Fir, Dissolving Pulp, Birch #1, Birch #2, Eucalyptus, Acacia, Mixed European Hardwood, Mixed Thai Hardwood, Tissue Dust, Cotton, Jeans, Abaca, Sisal, Bagasse, Kenaf, Miscanthus, Sorghum, Giant Reed and Flax.

In yet another embodiment of the first, second and third aspects, the product of hemicellulose mass fraction and fibre zero-span tensile index is about 15 to about 25 Nm/g, or is greater than 5 Nm/g, or is greater than 10 Nm/g, or is greater than 15 Nm/g, or is greater than 20 Nm/g, or is greater than 25 Nm/g, or is greater than 30 Nm/g, or is greater than 35 Nm/g, or is greater than 40 Nm/g, or is greater than 45 Nm/g, or is greater than 50 Nm/g.

In yet another embodiment of the first, second and third aspects, the product of hemicellulose mass fraction and fibre zero-span tensile index is about 20 Nm/g.

In another embodiment of the first, second and third aspects, the hemicellulose mass fraction of the fibrous substrate comprising cellulose is greater than 10%, or about 10% to about 25%, or about 10% to about 20% or greater than about 25%, or greater than about 30% or greater than about 35%, or greater than about 45% or greater than about 50%.

In another embodiment of the first, second and third aspects, the MFC fibre length are about 0.1 to 0.8 mm, or 0.1 to 0.25 mm, or 0.1 to 0.3 mm, or 0.1 to 0.4 mm, or 0.1 to 0.5 mm, or 0.1 to 0.6 mm, or 0.1 to 0.7 mm, or preferably, for example, 0.15 to 0.3 mm when produced in a stirred media mill at 300 kWh/t, 2.5% fibre solids and 47.5% MVC. In another embodiment, the MFC fibre length is greater than 0.25 mm.

In an embodiment of the first and second aspects, the grinding medium is removed at the completion of grinding.

In an embodiment of the first and second aspects, the MFC has a fibre steepness of from about 20 to about 50.

In an embodiment of the first and second aspects, the grinding medium is present in an amount of at least about 10% by volume of the aqueous environment.

In an embodiment of the first and second aspects, the mass ratio fibrous substrate to the inorganic particulate material are in a ratio of about 99.5:0.5 to about 0.5:99.5.

In an embodiment of the first and second aspects, the grinding is performed in a tower mill.

In an embodiment of the first and second aspects, the grinding is performed in a screened grinder. In an embodiment, the screened grinder is a stirred media detritor. In another embodiment, wherein the screened grinder comprises one or more screens having a nominal aperture size of at least about 250 μm. In another embodiment, the grinding is performed in a cascade of grinding vessels.

In an embodiment of the first and second aspects, the fibrous substrate comprising cellulose has a Canadian Standard freeness equal to or less than 450 cm³. In another embodiment of the first second aspects, the fibrous substrate comprising cellulose has a Canadian Standard freeness equal to or more than 450 cm³

In an embodiment of the first and second aspects, the grinding medium comprises particles having an average diameter in ranging from about 0.5 mm to about 6 mm. In another embodiment of the first and second aspects, the grinding medium comprises particles having an average diameter in ranging from about 6 mm to about 15 mm. In another embodiment, the grinding medium comprises particles having a specific gravity of at least about 2.5.

In an embodiment of the first and second aspects, the fibrous substrate comprising cellulose is present in the aqueous environment at initial solids content of at least about 5 wt %.

In an embodiment of the first and second aspects, the fibrous substrate comprising cellulose is present in the aqueous environment at an initial solids content of less than about 5 wt %, or less than about 4 wt %, or less than about 3 wt %, or less than about 2 wt %, or less than about 1.5 wt %, or less than about 1 wt %, or less than about 0.5 wt %.

In an embodiment of the first and second aspects, the total amount of energy used in the method is less than about 10,000 kWh per tonne of dry fibre in the fibrous substrate comprising cellulose, or less than about 5,000 kWh per tonne of dry fibre in the fibrous substrate comprising cellulose, or less than about 3,000 kWh per tonne of dry fibre in the fibrous substrate comprising cellulose, or less than about 2,500 kWh per tonne of dry fibre in the fibrous substrate comprising cellulose, or less than about 2,000 kWh per tonne of dry fibre in the fibrous substrate comprising cellulose.

In an embodiment of the first aspect, the inorganic particulate material is calcium carbonate. In another embodiment, the calcium carbonate is ground calcium carbonate. In another embodiment, the calcium carbonate is precipitated calcium carbonate. In yet another embodiment, the inorganic particulate material has a particle size distribution in which at least about 10% by weight of the particles have an e.s.d of less than 2 μm. In another embodiment, the inorganic particulate material has a particle size distribution in which at least about 20% by weight of the particles have an e.s.d of less than 2 μm. In a further embodiment, the inorganic particulate material has a particle size distribution in which at least about 30% by weight of the particles have an e.s.d of less than 2 μm. In another embodiment, the inorganic particulate material has a particle size distribution in which at least about 40% by weight of the particles have an e.s.d of less than 2 μm. In a further embodiment, the inorganic particulate material has a particle size distribution in which at least about 50% by weight of the particles have an e.s.d of less than 2 μm.

In an embodiment of the first and second aspects, the fibrous substrate comprising cellulose has a d₅₀ ranging from about 5 to μm about 500 μm, as measured by laser light scattering (e.g., Malvern Insitec L). In another embodiment, the fibrous substrate comprising cellulose has a d₅₀ equal to or less than about 200 μm, as measured by laser light scattering (e.g., Malvern Insitec L), or equal to or less than about 150 μm, as measured by laser light scattering, or equal to or less than about 100 μm, as measured by laser light scattering (e.g., Malvern Insitec L).

In another embodiment, the fibrous substrate comprising cellulose may be microfibrillated in the presence of an inorganic particulate material to obtain microfibrillated cellulose having a d₅₀ of equal to or less (as measured, for example, by a Malvern Insitec L) than about 400 μm, for example equal to or less than about 300 μm, or equal to or less than about 200 μm, or equal to or less than about 150 μm, or equal to or less than about 125 μm, or equal to or less than about 100 μm, or equal to or less than about 90 μm, or equal to or less than about 80 μm, or equal to or less than about 70 μm, or equal to or less than about 60 μm, or equal to or less than about 50 μm, or equal to or less than about 40 μm, or equal to or less than about 30 μm, or equal to or less than about 20 μm, or equal to or less than about 10 μm.

BRIEF DESCRIPTION OF THE FIGURES

For a more complete understanding of the principles disclosed herein, and the advantages thereof, reference is made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a plot of the MFC tensile index versus the product of the fibre zero-span tensile index and hemicellulose content, taken from Example 1. By using the product of the hemicellulose content and the fibre zero-span tensile index, a reasonably good prediction of the peak MFC tensile index could be made without requiring the production of MFC, see Example 1.

FIG. 2 is a plot of hemicellulose content (mass fraction) versus tensile index (Nm/g) of various fibrous substrates comprising cellulose at the selected energy input in Example 1. For this FIG. 2, crosses represent softwoods, circles represent hardwoods, diamonds represent cotton fibres, triangles represent leaf fibres, and squares represent other miscellaneous non-wood sources.

FIGS. 3A-D are Secondary electron SEM images of MFC made from (FIG. 3A) Bagasse (28% hemicellulose), (FIG. 3B) Nordic Pine (17% hemicellulose), (FIG. 3C) Dissolving Pulp (4% hemicellulose) and (FIG. 3D) Cotton (0% hemicellulose) processed to a specific energy input of 3000 kWh/t

FIG. 4 is a plot of tensile index versus the product of the hemicellulose content and the MFC length for the tested fibrous substrates comprising cellulose, as tested in Example 1.

FIG. 5 is a plot of the relationship between the zero-span tensile index of unground fibres and MFC length of the fibrous substrates comprising cellulose, as tested in Example 1.

FIG. 6 is a plot showing the relationship between the product of the hemicellulose content (mass fraction) and zero-span tensile index (Nm/g) of the unground fibres, with the resultant MFC tensile index when ground to the standard energy input identified by fibre source.

FIG. 7 is a plot of MFC tensile index versus total solids content of the slurry of microfibrillated cellulose and inorganic particulate materials at a 50 POP (50 wt. % fibre by dry mass). Fibre solids is found by multiplying the POP value by the total solids, which in the case of 50% POP halves the value, e.g. 4 wt. % total solids is equivalent to 2 wt. % fibre solids.

FIG. 8 is an illustration of the microfibril structure of cellulose showing the crystalline structure of cellulose chains, paracrystalline cellulose and hemicellulose coating cellulose and paracrystalline cellulose molecules reproduced from US DOE. 2005. Genomics: GTL Roadmap, DOE/SC-0090, U.S. Department of Energy Office of Science. (p. 27).

FIGS. 9A and 9B depict Birch fibres having a relatively high hemicellulose content which have been microfibrillated at 3,000 kWh/t (FIG. 9A) showing a highly fibrillated structure versus Cotton fibres, with little to no hemicellulose content (FIG. 9B) fibrillated at the same energy input.

FIGS. 10A-D are photographs by differential interference contrast microscopy of MFC prepared from different cellulose substrates, at a specific energy input of 3000 kWh/t, with the MFC having varying tensile index values. FIG. 10A depicts MFC prepared from Birch having a hemicellulose content of 25%, an MFC length of 0.23 mm and a tensile index of 12 Nm/g. FIG. 10B depicts MFC prepared from Flax having a hemicellulose content of 9%, an MFC length of 0.36 mm and a tensile index of 8 Nm/g. FIG. 10C depicts MFC prepared from Mixed European Hardwood, having a hemicellulose content of 22%, an MFC length of 15 mm and a tensile index of 7.5 Nm/g. FIG. 10D depicts MFC prepared from Cotton having a hemicellulose content of 0%, an MFC length of 15 mm and a tensile index of 4 Nm/g.

FIG. 11 is a plot of predicted tensile index using the modified Page Equation described in this specification for MFC, produced from 24 cellulose feed stocks.

FIG. 12 is a plot of predicted tensile index versus measured tensile index of MFC produced in Example 1 from 24 diverse sources of fibrous substrates comprising cellulose.

FIG. 13A is a graph of energy input versus tensile index of aqueous slurries of Birch fibres and inorganic particulate material at low solids contents of 1.5% total solids (0.75 fibre solids), 3% total solids (1.5% fibre solids) and 5% total solids (2.5% fibre solids). FIG. 13B is a graph of energy input (kWh/t) versus tear strength for slurries of Birch fibres and inorganic particulate material at low solids contents of 1.5% total solids (0.75 fibre solids) and 5% total solids (2.5% fibre solids).

FIGS. 14A and 14B are graphs of energy input (kWh/t) versus fibre length (in mm) and fibre width (in μm). FIG. 14A plots energy input (kWh/t) versus fibre length (in mm) for aqueous slurries of Birch fibres and inorganic particulate material at low solids contents of 1.5% total solids (0.75 fibre solids), 3% total solids (1.5% fibre solids) and 5% total solids (2.5% fibre solids). FIG. 14B plots energy input (kWh/t) versus fibre width (in mm) for aqueous slurries of Birch fibres and inorganic particulate material at low solids contents of 1.5% total solids (0.75 fibre solids), 3% total solids (1.5% fibre solids) and 5% total solids (2.5% fibre solids).

FIGS. 15A and 15B are graphs of energy input. FIG. 15A is a graph of energy input (kWh/t) versus the percentage of high aspect ratio fines detected in the fibrillation process of Example 2. FIG. 15B is graph of energy input (kWh/t) versus fibrillation percentage for aqueous slurries of Birch fibres and inorganic particulate material at low solids contents of 1.5% total solids (0.75 fibre solids), 3% total solids (1.5% fibre solids) and 5% total solids (2.5% fibre solids).

FIG. 16 is a differential interference microscopy photograph of pilot scale Birch fibres at 1.5% total solids (0.75% fibre solids) processed with an energy input of 1,000 kWh/t. The tensile index of the fibrillated fibres was recorded as 8.6 Nm/g.

FIG. 17 is a differential interference microscopy photograph of pilot scale Birch fibres at 1.5% total solids (0.75% fibre solids) processed with an energy input of 3,000 kWh/t. The tensile index of the microfibrillated cellulose was recorded as 12.8 Nm/g and 13.6 Nm/g, respectively.

FIG. 18 is a differential interference microscopy photograph of Birch fibres at 5% total solids (2.5% fibre solids) processed at pilot scale with an energy input of 3,000 kWh/t. The tensile index of the fibrillated fibres was recorded as 9.5 Nm/g.

FIGS. 19A and 19B are graphs of total solids content versus MFC tensile index (Nm/g) and Brookfield viscosity, respectively. FIG. 19A is a graph of total solids content (%) of aqueous suspensions comprising different fibrous substrates comprising cellulose and inorganic particulate material versus tensile index (Nm/g) determined for the microfibrillated cellulose produced from the different fibrous substrates comprising cellulose according to Example 3. FIG. 19B is a graph of total solids content (%) of the aqueous suspensions comprising different fibrous substrates comprising cellulose and inorganic particulate material versus the Brookfield viscosity measured at 10 rpm (mPas) determined for the microfibrillated cellulose produced from the different fibrous substrates comprising cellulose according to Example 3.

FIG. 20A and 20B are graphs of total solids (%) plotted against length-weighted fibre length (Lc(1)) (mm) (FIG. 20A) and Kink (1/m) measurements (FIG. 20B) for fibrous substrates comprising cellulose including Abaca, Cotton, Jeans, Kenaf, Bagasse, Tissue Dust and Mixed European Hardwood.

FIG. 21A and 21B are graphs of total solids contents (%) versus high aspect ratio fines (%) (FIG. 21A) and fibrillation (%) (FIG. 21B) for fibrous substrates comprising cellulose including Abaca, Cotton, Jeans, Kenaf, Bagasse, Tissue Dust and Mixed European Hardwood.

FIG. 22 is a graph of tensile index at 5% total solids (2.5% fibre solids) ground according to Example 3 plotted against the tensile index at 2% solids minus the tensile index at 8% solids for fibrous substrates comprising cellulose of Abaca, Cotton, Jeans, Kenaf, Bagasse, Tissue Dust, Mixed European Hardwood, Nordic Pine, Birch, Acacia Enzyme-Treated Nordic Pine and Eucalyptus.

FIG. 23 is a graph of specific energy input (kWh/t) versus tensile index (Nm/g) for 6 different fibrous substrates comprising cellulose, including Enzyme-Treated Nordic Pine, Abaca, Blue Roll (recycled Kimwipe tissue paper), Eucalyptus, Nordic Pine, Acacia, Birch and Cotton.

DETAILED DESCRIPTION OF THE INVENTION

The titles, headings and subheadings provided herein should not be interpreted as limiting the various aspects of the disclosure. Accordingly, the terms defined below are more fully defined by reference to the specification in its entirety. All references cited herein are incorporated by reference in their entirety.

Unless otherwise defined, scientific and technical terms used herein shall have the meanings that are commonly understood by those of ordinary skill in the art. Further, unless otherwise required by context, singular terms shall include pluralities and plural terms shall include the singular.

In this application, the use of “or” means “and/or” unless stated otherwise. In the context of a multiple dependent claim, the use of “or” refers back to more than one preceding independent or dependent claim in the alternative only.

It is further noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the,” and any singular use of any word, include plural referents unless expressly and unequivocally limited to one referent.

The instant invention is most clearly understood with reference to the following definitions:

The term “about” is used herein to mean approximately, in the region of, roughly, or around. When the term “about” is used in conjunction with a numerical range, it modifies that range by extending the boundaries above and below the numerical values set forth. In general, the term “about” is used herein to modify a numerical value above and below the stated value by a variance of 10%.

As used herein, the terms “comprising” (and any form of comprising, such as “comprise”, “comprises”, and “comprised”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “includes” and “include”), or “containing” (and any form of containing, such as “contains” and “contain”), are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. Additionally, a term that is used in conjunction with the term “comprising” is also understood to be able to be used in conjunction with the term “consisting of” or “consisting essentially of.”

As used herein, the term “include” and its grammatical variants are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that can be substituted or added to the listed items.

As used herein, the phrase “integer from X to Y” means any integer that includes the endpoints. For example, the phrase “integer from 1 to 5” means 1, 2, 3, 4, or 5.

The present invention is related to modifications, for example, improvements, to the methods and compositions described in WO-A-2010/131016, the entire contents of which are hereby incorporated by reference.

WO-A-2010/131016 discloses a process for preparing microfibrillated cellulose comprising microfibrillating, e.g., by grinding, a fibrous material comprising cellulose, optionally in the presence of grinding medium and inorganic particulate material. When used as a filler in paper, for example, as a replacement or partial replacement for a conventional mineral filler, the microfibrillated cellulose obtained by said process, optionally in combination with inorganic particulate material, was unexpectedly found to improve the burst strength properties of the paper. That is, relative to a paper filled with exclusively mineral filler, paper filled with the microfibrillated cellulose was found to have improved burst strength. In other words, the microfibrillated cellulose filler was found to have paper burst strength enhancing attributes. In one particularly advantageous embodiment of that invention, the fibrous material comprising cellulose was ground in the presence of a grinding medium, optionally in combination with inorganic particulate material, to obtain microfibrillated cellulose having a fibre steepness of from 20 to about 50.

While the microfibrillated cellulose obtainable by the processes described in WO-A-2010/131016 has been shown to have advantageous paper burst strength enhancing attributes, it would be desirable to be able to modify, for example, further improve, one or more paper property enhancing attributes of microfibrillated cellulose, for example, the paper burst strength enhancing attributes of microfibrillated cellulose and/or the tensile properties of the MFC.

The method described in WO-A-2010/131016 comprises a step of microfibrillating a fibrous substrate comprising cellulose by grinding in the presence of a particulate grinding medium which is to be removed after the completion of grinding. By “microfibrillating” is meant a process in which microfibrils of cellulose are liberated or partially liberated as individual species or as small aggregates as compared to the fibres of the pre-microfibrillated pulp. Typical cellulose fibres (i.e., pre-microfibrillated pulp) suitable for use in papermaking include larger aggregates of hundreds or thousands of individual cellulose fibrils. By microfibrillating the cellulose, particular characteristics and properties, including the characteristics and properties described herein, are imparted to the microfibrillated cellulose and the compositions comprising the microfibrillated cellulose.

The Fibrous Substrate Comprising Cellulose.

The fibrous substrate comprising cellulose (variously referred to herein as “fibrous substrate comprising cellulose,” “cellulose fibres,” “fibrous cellulose feedstock,” “cellulose feedstock” and “cellulose-containing fibres (or fibrous,” etc.) may be derived from any suitable source, such as wood, grasses (e.g., sugarcane, bamboo) or rags (e.g., textile waste, cotton, hemp or flax). The fibrous substrate comprising cellulose may be in the form of a pulp (i.e., a suspension of cellulose fibres in water), which may be prepared by any suitable chemical or mechanical treatment, or combination thereof. For example, the pulp may be a chemical pulp, or a chemithermomechanical pulp, or a mechanical pulp, or a recycled pulp, or a papermill broke, or a papermill waste stream, or waste from a papermill, or a combination thereof. The cellulose pulp may be beaten (for example in a Valley beater) and/or otherwise refined (for example, processing in a conical or plate refiner) to any predetermined freeness, reported in the art as Canadian standard freeness (CSF) in cm³. CSF means a value for the freeness or drainage rate of pulp measured by the rate that a suspension of pulp may be drained, and this test is carried out according to the T 227 cm-09 TAPPI standard. For example, the cellulose pulp may have a Canadian standard freeness of about 10 cm³ or greater prior to being microfibrillated. The cellulose pulp may have a CSF of about 700 cm³ or less, for example, equal to or less than about 650 cm³, or equal to or less than about 600 cm³, or equal to or less than about 550 cm³, or equal to or less than about 500 cm³, or equal to or less than about 450 cm³, or equal to or less than about 400 cm³, or equal to or less than about 350 cm³, or equal to or less than about 300 cm³, or equal to or less than about 250 cm³, or equal to or less than about 200 cm³, or equal to or less than about 150 cm³, or equal to or less than about 100 cm³, or equal to or less than about 50 cm³. The cellulose pulp may then be dewatered by methods well known in the art, for example, the pulp may be filtered through a screen in order to obtain a wet sheet comprising at least about 10% solids, for example at least about 15% solids, or at least about 20% solids, or at least about 30% solids, or at least about 40% solids. The pulp may be utilised in an unrefined state, that is to say without being beaten or dewatered, or otherwise refined.

The fibrous substrate comprising cellulose may be added to a grinding vessel fibrous substrate comprising cellulose in a dry state. For example, a dry paper broke may be added directly to the grinder vessel. The aqueous environment in the grinder vessel will then facilitate the formation of a pulp.

The Inorganic Particulate Material.

The inorganic particulate material, when present, may, for example, be an alkaline earth metal carbonate or sulphate, such as calcium carbonate, magnesium carbonate, dolomite, gypsum, a hydrous kandite day such as kaolin, halloysite or ball clay, an anhydrous (calcined) kandite clay such as metakaolin or fully calcined kaolin, talc, mica, perlite or diatomaceous earth, or magnesium hydroxide, or aluminium trihydrate, or combinations thereof.

A preferred inorganic particulate material for use in the method is calcium carbonate. Hereafter, the invention may tend to be discussed in terms of calcium carbonate, and in relation to aspects where the calcium carbonate is processed and/or treated. The invention should not be construed as being limited to such embodiments.

The particulate calcium carbonate used in the present invention may be obtained from a natural source by grinding. Ground calcium carbonate (GCC) is typically obtained by crushing and then grinding a mineral source such as chalk, marble or limestone, which may be followed by a particle size classification step, in order to obtain a product having the desired degree of fineness. Other techniques such as bleaching, flotation and magnetic separation may also be used to obtain a product having the desired degree of fineness and/or colour. The particulate solid material may be ground autogenously, i.e. by attrition between the particles of the solid material themselves, or, alternatively, in the presence of a particulate grinding medium comprising particles of a different material from the calcium carbonate to be ground. These processes may be carried out with or without the presence of a dispersant and biocides, which may be added at any stage of the process.

Precipitated calcium carbonate (PCC) may be used as the source of particulate calcium carbonate in the present invention, and may be produced by any of the known methods available in the art. TAPPI Monograph Series No 30, “Paper Coating Pigments”, pages 34-35 describes the three main commercial processes for preparing precipitated calcium carbonate which is suitable for use in preparing products for use in the paper industry, but may also be used in the practice of the present invention. In all three processes, a calcium carbonate feed material, such as limestone, is first calcined to produce quicklime, and the quicklime is then slaked in water to yield calcium hydroxide or milk of lime. In the first process, the milk of lime is directly carbonated with carbon dioxide gas. This process has the advantage that no by-product is formed, and it is relatively easy to control the properties and purity of the calcium carbonate product. In the second process the milk of lime is contacted with soda ash to produce, by double decomposition, a precipitate of calcium carbonate and a solution of sodium hydroxide. The sodium hydroxide may be substantially completely separated from the calcium carbonate if this process is used commercially. In the third main commercial process the milk of lime is first contacted with ammonium chloride to give a calcium chloride solution and ammonia gas. The calcium chloride solution is then contacted with soda ash to produce by double decomposition precipitated calcium carbonate and a solution of sodium chloride. The crystals can be produced in a variety of different shapes and sizes, depending on the specific reaction process that is used. The three main forms of PCC crystals are aragonite, rhombohedral and scalenohedral, all of which are suitable for use in the present invention, including mixtures thereof

Wet grinding of calcium carbonate involves the formation of an aqueous suspension of the calcium carbonate which may then be ground, optionally in the presence of a suitable dispersing agent. Reference may be made to, for example, EP-A-614948 (the contents of which are incorporated by reference in their entirety) for more information regarding the wet grinding of calcium carbonate.

In some circumstances, minor additions of other minerals may be included, for example, one or more of kaolin, calcined kaolin, wollastonite, bauxite, talc or mica, could also be present.

When the inorganic particulate material of the present invention is obtained from naturally occurring sources, it may be that some mineral impurities will contaminate the ground material. For example, naturally occurring calcium carbonate can be present in association with other minerals. Thus, in some embodiments, the inorganic particulate material includes an amount of impurities. In general, however, the inorganic particulate material used in the invention will contain less than about 5% by weight, preferably less than about 1% by weight, of other mineral impurities.

The inorganic particulate material used during the microfibrillating step of the method of the present invention will preferably have a particle size distribution in which at least about 10% by weight of the particles have an e.s.d of less than 2 μm, for example, at least about 20% by weight, or at least about 30% by weight, or at least about 40% by weight, or at least about 50% by weight, or at least about 60% by weight, or at least about 70% by weight, or at least about 80% by weight, or at least about 90% by weight, or at least about 95% by weight, or about 100% of the particles have an e.s.d of less than 2 μm.

Unless otherwise stated, particle size properties referred to herein for the inorganic particulate materials are as measured in a well-known manner by sedimentation of the particulate material in a fully dispersed condition in an aqueous medium using a Sedigraph 5100 machine as supplied by Micromeritics Instruments Corporation, Norcross, Ga., USA (telephone: +1 770 662 3620; web-site: www.micromeritics.com), referred to herein as a “Micromeritics Sedigraph 5100 unit”. Such a machine provides measurements and a plot of the cumulative percentage by weight of particles having a size, referred to in the art as the equivalent spherical diameter (e.s.d), less than given e.s.d values. The mean particle size d₅₀ is the value determined in this way of the particle e.s.d at which there are 50% by weight of the particles which have an equivalent spherical diameter less than that d₅₀ value.

Alternatively, where stated, the particle size properties referred to herein for the inorganic particulate materials are as measured by the well-known conventional method employed in the art of laser light scattering, using a Malvern Mastersizer S machine as supplied by Malvern Instruments Ltd (or by other methods which give essentially the same result). In the laser light scattering technique, the size of particles in powders, suspensions and emulsions may be measured using the diffraction of a laser beam, based on an application of Mie theory. Such a machine provides measurements and a plot of the cumulative percentage by volume of particles having a size, referred to in the art as the ‘equivalent spherical diameter’ (e.s.d), less than given e.s.d values. The mean particle size d₅₀ is the value determined in this way of the particle e.s.d at which there are 50% by volume of the particles which have an equivalent spherical diameter less than that d₅₀ value.

In another embodiment, the inorganic particulate material used during the microfibrillating step of the method of the present invention will preferably have a particle size distribution, as measured using a Malvern Mastersizer S machine, in which at least about 10% by volume of the particles have an e.s.d of less than 2 μm, for example, at least about 20% by volume, or at least about 30% by volume, or at least about 40% by volume, or at least about 50% by volume, or at least about 60% by volume, or at least about 70% by volume, or at least about 80% by volume, or at least about 90% by volume, or at least about 95% by volume, or about 100% of the particles by volume have an e.s.d of less than 2 μm.

Unless otherwise stated, particle size properties of the microfibrillated cellulose materials are as are as measured by the well-known conventional method employed in the art of laser light scattering, using a Malvern Insitec L machine as supplied by Malvern Instruments Ltd (or by other methods which give essentially the same result).

Details of the procedure used to characterise the particle size distributions of mixtures of inorganic particle material and microfibrillated cellulose using a Malvern Mastersizer S machine are provided below.

Another preferred inorganic particulate material for use is kaolin clay. Hereafter, this section of the specification may tend to be discussed in terms of kaolin, and in relation to aspects where the kaolin is processed and/or treated. The invention should not be construed as being limited to such embodiments. Thus, in some embodiments, kaolin is used in an unprocessed form.

Kaolin clay used in this invention may be a processed material derived from a natural source, namely raw natural kaolin clay mineral. The processed kaolin clay may typically contain at least about 50% by weight kaolinite. For example, most commercially processed kaolin clays contain greater than about 75% by weight kaolinite and may contain greater than about 90%, in some cases greater than about 95% by weight of kaolinite.

Kaolin clay used in the present invention may be prepared from the raw natural kaolin clay mineral by one or more other processes which are well known to those skilled in the art, for example by known refining or beneficiation steps.

For example, the clay mineral may be bleached with a reductive bleaching agent, such as sodium hydrosulfite. If sodium hydrosulfite is used, the bleached clay mineral may optionally be dewatered, and optionally washed and again optionally dewatered, after the sodium hydrosulfite bleaching step.

The clay mineral may be treated to remove impurities, e.g. by flocculation, flotation, or magnetic separation techniques well known in the art. Alternatively the clay mineral used in the first aspect of the invention may be untreated in the form of a solid or as an aqueous suspension.

The process for preparing the particulate kaolin clay used in the present invention may also include one or more comminution steps, e.g., grinding or milling. Light comminution of a coarse kaolin is used to give suitable delamination thereof. The comminution may be carried out by use of beads or granules of a plastic (e.g. nylon), sand or ceramic grinding or milling aid. The coarse kaolin may be refined to remove impurities and improve physical properties using well known procedures. The kaolin clay may be treated by a known particle size classification procedure, e.g., screening and centrifuging (or both), to obtain particles having a desired d₅₀ value or particle size distribution.

The Microfibrillating Process

Generally, the microfibrillating process, in one aspect, comprises microfibrillating a fibrous substrate comprising cellulose in the presence of an inorganic particulate material. According to particular embodiments of the present methods, the microfibrillating step is conducted in the presence of an inorganic particulate material which acts as a microfibrillating agent.

By microfibrillating is meant a process in which microfibrils of cellulose are liberated or partially liberated as individual species or as smaller aggregates as compared to the fibres of the pre-microfibrillated pulp. Typical cellulose fibres (i.e., pre-microfibrillated pulp) suitable for use in papermaking include larger aggregates of hundreds or thousands of individual cellulose microfibrils. By microfibrillating the cellulose, particular characteristics and properties, including but not limited to the characteristic and properties described herein, are imparted to the microfibrillated cellulose and the compositions including the microfibrillated cellulose.

The step of microfibrillating may be carried out in any suitable apparatus, including but not limited to a refiner. In one embodiment, the microfibrillating step is conducted in a grinding vessel under wet-grinding conditions. In a particular embodiment, the grinding vessel is a screened grinding vessel. In another embodiment, the screened grinding vessel is a stirred media detritor. In another embodiment, the microfibrillating step is carried out in a homogenizer. Each of these embodiments is described in greater detail below.

Wet-Grinding

The grinding is suitably performed in a conventional manner. The grinding may be an attrition grinding process in the presence of a particulate grinding medium, or may be an autogenous grinding process, i.e., one in the absence of a grinding medium. By grinding medium is meant a medium other than the inorganic particulate material which is co-ground with the fibrous substrate comprising cellulose.

The particulate grinding medium, when present, may be of a natural or a synthetic material. The grinding medium may, for example, comprise balls, beads or pellets of any hard mineral, ceramic or metallic material. Such materials may include, for example, alumina, zirconia, zirconium silicate, aluminium silicate or the mullite-rich material which is produced by calcining kaolinitic clay at a temperature in the range of from about 1300° C. to about 1800° C. For example, in some embodiments a Carbolite® grinding media is preferred. Alternatively, particles of natural sand of a suitable particle size may be used.

Generally, the type of and particle size of grinding medium to be selected for use in the invention may be dependent on the properties, such as, e.g., the particle size of, and the chemical composition of, the feed suspension of material to be ground. Preferably, the particulate grinding medium comprises particles having an average diameter in the range of from about 0.1 mm to about 6.0 mm and, more preferably, in the range of from about 0.2 mm to about 4.0 mm. Coarser media of 6-15 mm may alternatively be utilized in certain embodiments. The grinding medium (or media) may be present in an amount up to about 70% by volume of the charge. The grinding media may be present in amount of at least about 10% by volume of the charge, for example, at least about 20% by volume of the charge, or at least about 30% by volume of the charge, or at least about 40% by volume of the charge, or at least about 50% by volume of the charge, or at least about 60% by volume of the charge.

The grinding may be carried out in one or more stages. For example, a coarse inorganic particulate material may be ground in the grinder vessel to a predetermined particle size distribution, after which the fibrous material comprising cellulose is added and the grinding continued until the desired level of microfibrillation has been obtained. The coarse inorganic particulate material used in accordance with the first aspect of this invention initially may have a particle size distribution in which less than about 20% by weight of the particles have an e.s.d of less than 2 μm, for example, less than about 15% by weight, or less than about 10% by weight of the particles have an e.s.d. of less than 2 μm.

The coarse inorganic particulate material may be wet or dry ground in the absence or presence of a grinding medium. In the case of a wet grinding stage, the coarse inorganic particulate material is preferably ground in an aqueous suspension in the presence of a grinding medium. In such a suspension, the coarse inorganic particulate material may preferably be present in an amount of from about 5% to about 85% by weight of the suspension; more preferably in an amount of from about 20% to about 80% by weight of the suspension. Most preferably, the coarse inorganic particulate material may be present in an amount of about 30% to about 75% by weight of the suspension. As described above, the coarse inorganic particulate material may be ground to a particle size distribution such that at least about 10% by weight of the particles have an e.s.d of less than 2 μm, for example, at least about 20% by weight, or at least about 30% by weight, or at least about 40% by weight, or at least about 50% by weight, or at least about 60% by weight, or at least about 70% by weight, or at least about 80% by weight, or at least about 90% by weight, or at least about 95% by weight, or about 100% by weight of the particles, have an e.s.d of less than 2 μm, after which the cellulose pulp is added and the two components are co-ground to microfibrillate the fibres of the cellulose pulp. In another embodiment, the coarse inorganic particulate material is ground to a particle size distribution, as measured using a Malvern Mastersizer S machine such that at least about 10% by volume of the particles have an e.s.d of less than 2 μm, for example, at least about 20% by volume, or at least about 30% by volume or at least about 40% by volume, or at least about 50% by volume, or at least about 60% by volume, or at least about 70% by volume, or at least about 80% by volume, or at least about 90% by volume, or at least about 95% by volume, or about 100% by volume of the particles, have an e.s.d of less than 2 μm, after which the cellulose pulp is added and the two components are co-ground to microfibrillate the fibres of the cellulose pulp.

In one embodiment, the mean particle size (d₅₀) of the inorganic particulate material is reduced during the co-grinding process. For example, the d₅₀ of the inorganic particulate material may be reduced by at least about 10% (as measured by a Malvern Mastersizer S machine), for example, the d₅₀ of the inorganic particulate material may be reduced by at least about 20%, or reduced by at least about 30%, or reduced by at least about 50%, or reduced by at least about 50%, or reduced by at least about 60%, or reduced by at least about 70%, or reduced by at least about 80%, or reduced by at least about 90%. For example, an inorganic particulate material having a d₅₀ of 2.5 μm prior to co-grinding and a d₅₀ of 1.5 μm post co-grinding will have been subject to a 40% reduction in particle size. In some embodiments, the mean particle size of the inorganic particulate material is not significantly reduced during the co-grinding process. By ‘not significantly reduced’ is meant that the d₅₀ the inorganic particulate material is reduced by less than about 10%, for example, the d₅₀ of the inorganic particulate material is reduced by less than about 5%.

The fibrous substrate comprising cellulose may be microfibrillated in the presence of an inorganic particulate material to obtain microfibrillated cellulose having a d₅₀ ranging from about 5 μm to about 500 μm, as measured by laser light scattering. The fibrous substrate comprising cellulose may be microfibrillated in the presence of an inorganic particulate material to obtain microfibrillated cellulose having a d₅₀ of equal to or less than about 400 μm, for example equal to or less than about 300 μm, or equal to or less than about 200 μm, or equal to or less than about 150 μm, or equal to or less than about 125 μm, or equal to or less than about 100 μm, or equal to or less than about 90 μm, or equal to or less than about 80 μm, or equal to or less than about 70 μm, or equal to or less than about 60 μm, or equal to or less than about 50 μm, or equal to or less than about 40 μm, or equal to or less than about 30 μm, or equal to or less than about 20 μm, or equal to or less than about 10 μm.

The fibrous substrate comprising cellulose may be microfibrillated in the presence of an inorganic particulate material to obtain microfibrillated cellulose having a modal fibre particle size ranging from about 0.1-500 μm and a modal inorganic particulate material particle size ranging from 0.25-20 μm. The fibrous substrate comprising cellulose may be microfibrillated in the presence of an inorganic particulate material to obtain microfibrillated cellulose having a modal fibre particle size of at least about 0.5 μm, for example at least about 10 μm, or at least about 50 μm, or at least about 100 μm, or at least about 150 μm, or at least about 200 μm, or at least about 300 μm, or at least about 400 μm.

The fibrous substrate comprising cellulose may be microfibrillated in the presence of an inorganic particulate material to obtain microfibrillated cellulose having a fibre steepness equal to or greater than about 10, as measured by Malvern. Fibre steepness (i.e., the steepness of the particle size distribution of the fibres) is determined by the following formula:

Steepness=100×(d ₃₀ /d ₇₀)

The microfibrillated cellulose may have a fibre steepness equal to or less than about 100. The microfibrillated cellulose may have a fibre steepness equal to or less than about 75, or equal to or less than about 50, or equal to or less than about 40, or equal to or less than about 30. The microfibrillated cellulose may have a fibre steepness from about 20 to about 50, or from about 25 to about 40, or from about 25 to about 35, or from about 30 to about 40.

The particle size distribution may be calculated from Mie theory and the output as a differential volume-based distribution. The presence of two distinct peaks can be interpreted as arising from the mineral (finer peak) and fibre (coarser peak).

The finer mineral peak can be fitted to the measured data points and subtracted mathematically from the distribution to leave the fibre peak, which can be converted to a cumulative distribution. Similarly, the fibre peak can be subtracted mathematically from the original distribution to leave the mineral peak, which can also be converted to a cumulative distribution. Both these cumulative curves may then be used to calculate the mean particle size (d₅₀) and the steepness of the distribution (d₃₀/d₇₀×100). The differential curve may then be used to find the modal particle size for both the mineral and fibre fractions.

The grinding is suitably performed in a grinding vessel, such as a tumbling mill (e.g., rod, ball and autogenous), a stirred mill (e.g., Sala Agitated Mill or IsaMill), a tower mill, a stirred media detritor (SMD), or a grinding vessel comprising rotating parallel grinding plates between which the feed to be ground is fed.

In an embodiment, the grinding is performed in a screened grinder, preferably a stirred media detritor. The screened grinder may comprise one or more screen(s) having a nominal aperture size of at least about 250 μm, for example, the one or more screens may have a nominal aperture size of at least about 300 μm, or at least about 350 μm, or at least about 400 μm, or at least about 450 μm, or at least about 500 μm, or at least about 550 μm, or at least about 600 μm, or at least about 650 μm, or at least about 700 μm, or at least about 750 μm, or at least about 800 μm, or at least about 850 μm, or at or least about 900 μm, or at least about 1000 μm or at least about 1250 μm, or at least about 1500 μm. The screen sizes noted immediately above are applicable to use of a tower mill embodiments as well.

As noted above, the grinding may be performed in the presence of a grinding medium. In an embodiment, the grinding medium is a coarse media comprising particles having an average diameter in the range of from about 1 mm to about 6 mm, for example about 2 mm, or about 3 mm, or about 4 mm, or about 5 mm. In alternative embodiments, media may have an average diameter in the range of 6 mm to 15 mm.

In another embodiment, the grinding media has a specific gravity of at least about 2.5, for example, at least about 3, or at least about 3.5, or at least about 4.0, or at least about 4.5, or least about 5.0, or at least about 5.5, or at least about 6.0.

In another embodiment, the grinding media comprises particles having an average diameter in the range of from about 1 mm to about 6 mm and has a specific gravity of at least about 2.5.

In another embodiment, the grinding media comprises particles having an average diameter of about 3 mm and specific gravity of about 2.7.

As described above, the grinding medium (or media) may present in an amount up to about 70% by volume of the charge. The grinding media may be present in amount of at least about 10% by volume of the charge, for example, at least about 20% by volume of the charge, or at least about 30% by volume of the charge, or at least about 40% by volume of the charge, or at least about 50% by volume of the charge, or at least about 60% by volume of the charge.

In one embodiment, the grinding medium is present in amount of about 50% by volume of the charge. By charge' is meant the composition which is the feed fed to the grinder vessel. The charge includes of water, grinding media, fibrous substrate comprising cellulose and inorganic particulate material, and any other optional additives as described herein. The use of a relatively coarse and/or dense media has the advantage of improved (i.e., faster) sediment rates and reduced media carry over through the quiescent zone and/or classifier and/or screen(s).

A further advantage in using relatively coarse grinding media is that the mean particle size (d₅₀) of the inorganic particulate material may not be significantly reduced during the grinding process such that the energy imparted to the grinding system is primarily expended in microfibrillating the fibrous substrate comprising cellulose.

A further advantage in using relatively coarse screens is that a relatively coarse or dense grinding media can be used in the microfibrillating step. In addition, the use of relatively coarse screens (i.e., having a nominal aperture of least about 250 μm) allows a relatively high solids product to be processed and removed from the grinder, which allows a relatively high solids feed (comprising fibrous substrate comprising cellulose and inorganic particulate material) to be processed in an economically viable process. As discussed below, it has been found that a feed having a high initial solids content is desirable in terms of energy efficiency. Further, it has also been found that product produced (at a given energy) at lower solids has a coarser particle size distribution.

As discussed in the ‘Background’ section above, the present invention seeks to address the problem of preparing microfibrillated cellulose economically on an industrial scale.

Thus, in accordance with one embodiment, the fibrous substrate comprising cellulose and inorganic particulate material are present in the aqueous environment at an initial solids content of at least about 4 wt. %, of which at least about 2% by weight is fibrous substrate comprising cellulose. In some embodiments, the initial solids content may be at least about 0.25 wt. %, 0.5 wt. %, 1 wt. %, 1.5 wt. %, 2 wt. %, 2,5 wt. %, 3 wt. %, 4 wt. %, 5 wt. %. In some embodiments the initial solids content may be at least about 6 wt. %, 7 wt. %, 8 wt. %, 9 wt. % or about 10 wt. %. At least about 5% by weight of the initial solids content may be fibrous substrate comprising cellulose.

In another embodiment, the grinding is performed in a cascade of grinding vessels, one or more of which may comprise one or more grinding zones. For example, the fibrous substrate comprising cellulose and the inorganic particulate material may be ground in a cascade of two or more grinding vessels, for example, a cascade of three or more grinding vessels, or a cascade of four or more grinding vessels, or a cascade of five or more grinding vessels, or a cascade of six or more grinding vessels, or a cascade of seven or more grinding vessels, or a cascade of eight or more grinding vessels, or a cascade of nine or more grinding vessels in series, or a cascade comprising up to ten grinding vessels. The cascade of grinding vessels may be operatively linked in series or parallel or a combination of series and parallel. The output from and/or the input to one or more of the grinding vessels in the cascade may be subjected to one or more screening steps and/or one or more classification steps.

The total energy expended in a microfibrillation process may be apportioned equally across each of the grinding vessels in the cascade. Alternatively, the energy input may vary between some or all of the grinding vessels in the cascade.

A person skilled in the art will understand that the energy expended per vessel may vary between vessels in the cascade depending on the amount of fibrous substrate being microfibrillated in each vessel, and optionally the speed of grind in each vessel, the duration of grind in each vessel, the type of grinding media in each vessel and the type and amount of inorganic particulate material. The grinding conditions may be varied in each vessel in the cascade in order to control the particle size distribution of both the microfibrillated cellulose and the inorganic particulate material. For example, the grinding media size may be varied between successive vessels in the cascade in order to reduce grinding of the inorganic particulate material and to target grinding of the fibrous substrate comprising cellulose.

In an embodiment the grinding is performed in a closed circuit. In another embodiment, the grinding is performed in an open circuit. The grinding may be performed in batch mode. The grinding may be performed in a re-circulating batch mode.

As described above, the grinding circuit may include a pre-grinding step in which coarse inorganic particulate ground in a grinder vessel to a predetermined particle size distribution, after which fibrous material comprising cellulose is combined with the pre-ground inorganic particulate material and the grinding continued in the same or different grinding vessel until the desired level of microfibrillation has been obtained.

As the suspension of material to be ground may be of a relatively high viscosity, a suitable dispersing agent may preferably be added to the suspension prior to grinding. The dispersing agent may be, for example, a water soluble condensed phosphate, polysilicic acid or a salt thereof, or a polyelectrolyte, for example a water soluble salt of a poly(acrylic acid) or of a poly(methacrylic acid) having a number average molecular weight not greater than 80,000. The amount of the dispersing agent used would generally be in the range of from 0.1 to 2.0% by weight, based on the weight of the dry inorganic particulate solid material. The suspension may suitably be ground at a temperature in the range of from 4° C. to 100° C.

Other additives which may be included during the microfibrillation step include: carboxymethyl cellulose, amphoteric carboxymethyl cellulose, oxidising agents, 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO), TEMPO derivatives, and wood degrading enzymes.

The pH of the suspension of material to be ground may be about 7 or greater than about 7 (i.e., basic), for example, the pH of the suspension may be about 8, or about 9, or about 10, or about 11. The pH of the suspension of material to be ground may be less than about 7 (i.e., acidic), for example, the pH of the suspension may be about 6, or about 5, or about 4, or about 3.

The pH of the suspension of material to be ground may be adjusted by addition of an appropriate amount of acid or base. Suitable bases included alkali metal hydroxides, such as, for example NaOH. Other suitable bases are sodium carbonate and ammonia. Suitable acids included inorganic acids, such as hydrochloric and sulphuric acid, or organic acids. An exemplary acid is orthophosphoric acid.

The amount of inorganic particulate material and cellulose pulp in the mixture to be co-ground may vary in a ratio of from about 99.5:0.5 to about 0.5:99.5, based on the dry weight of inorganic particulate material and the amount of dry fibre in the pulp, for example, a ratio of from about 99.5:0.5 to about 50:50 based on the dry weight of inorganic particulate material and the amount of dry fibre in the pulp. For example, the ratio of the amount of inorganic particulate material and dry fibre may be from about 99.5:0.5 to about 70:30. In an embodiment, the ratio of inorganic particulate material to dry fibre is about 80:20, or for example, about 85:15, or about 90:10, or about 91:9, or about 92:8, or about 93:7, or about 94:6, or about 95:5, or about 96:4, or about 97:3, or about 98:2, or about 99:1. In another embodiment, the weight ratio of inorganic particulate material and dry fibre may be about 50:50. In a preferred embodiment, the weight ratio of inorganic particulate material to dry fibre is about 95:5. In yet another embodiment the ratio may be expressed as 50% percentage of pulp (50 POP) or 20% percentage of pulp (20 POP). In another preferred embodiment, the weight ratio of inorganic particulate material to dry fibre is about 90:10. In another preferred embodiment, the weight ratio of inorganic particulate material to dry fibre is about 85:15. In another preferred embodiment, the weight ratio of inorganic particulate material to dry fibre is about 80:20.

The total energy input in a typical grinding process to obtain the desired aqueous suspension composition may typically be between about 100 and 1500 kWht⁻¹ based on the total dry weight of the inorganic particulate filler. The total energy input may be less than about 1000 kWht⁻¹, for example, less than about 800 kWht⁻¹, less than about 600 kWht⁻¹, less than about 500 kWht⁻¹, less than about 400 kWht⁻¹, less than about 300 kWht⁻¹, or less than about 200 kWht⁻¹.

A cellulose pulp can be microfibrillated at relatively low energy input when it is co-ground in the presence of an inorganic particulate material. The total energy input per tonne of dry fibre in the fibrous substrate comprising cellulose will be less than about 10,000 kWht⁻¹, for example, less than about 9000 kWht⁻¹, or less than about 8000 kWht⁻¹, or less than about 7000 kWht⁻¹, or less than about 6000 kWht⁻¹, or less than about 5000 kWht⁻¹, for example less than about 4000 kWht⁻¹, less than about 3000 kWht⁻¹, less than about 2000 kWht⁻¹, less than about 1500 kWht⁻¹, less than about 1200 kWht⁻¹, less than about 1000 kWht⁻¹, or less than about 800 kWht⁻¹. The total energy input varies depending on the amount of dry fibre in the fibrous substrate being microfibrillated, and optionally the speed of grind and the duration of grind.

Microfibrillation in the Absence of Grindable Inorganic Particulate Material

In another aspect, microfibrillation of cellulose fibres may be performed in a process comprising a step of microfibrillating a fibrous substrate comprising cellulose in an aqueous environment by grinding in the presence of a grinding medium which is to be removed after the completion of grinding, wherein the grinding is performed in a tower mill or a screened grinder, including a stirred media detritor, and wherein the grinding is carried out in the absence of grindable inorganic particulate material. A grindable inorganic particulate material is a material which would be ground in the presence of the grinding medium.

The particulate grinding medium may be of a natural or a synthetic material. The grinding medium may, for example, comprise balls, beads or pellets of any hard mineral, ceramic or metallic material. Such materials may include, for example, alumina, zirconia, zirconium silicate, aluminium silicate or the mullite-rich material which is produced by calcining kaolinitic clay at a temperature in the range of from about 1300° C. to about 1800° C. For example, in some embodiments a Carbolite® grinding media is preferred. Alternatively, particles of natural sand of a suitable particle size may be used.

Generally, the type of and particle size of grinding medium to be selected for use in the invention may be dependent on the properties, such as, e.g., the particle size of, and the chemical composition of, the feed suspension of material to be ground. Preferably, the particulate grinding medium comprises particles having an average diameter in the range of from about 0.5 mm to about 6 mm. In one embodiment, the particles have an average diameter of at least about 3 mm. In yet other embodiments, the average diameter range may be 6 mm to 15 mm.

The grinding medium may comprise particles having a specific gravity of at least about 2.5. The grinding medium may comprise particles have a specific gravity of at least about 3, or least about 4, or least about 5, or at least about 6.

The grinding medium (or media) may be present in an amount up to about 70% by volume of the charge. The grinding media may be present in amount of at least about 10% by volume of the charge, for example, at least about 20% by volume of the charge, or at least about 30% by volume of the charge, or at least about 40% by volume of the charge, or at least about 50% by volume of the charge, or at least about 60% by volume of the charge.

The fibrous substrate comprising cellulose may be microfibrillated in the presence of an inorganic particulate material to obtain microfibrillated cellulose having a d₅₀ ranging from about 5 μm to about 500 μm, as measured by laser light scattering. The fibrous substrate comprising cellulose may be microfibrillated in the presence of an inorganic particulate material to obtain microfibrillated cellulose having a d₅₀ of equal to or less than about 400 μm, for example equal to or less than about 300 μm, or equal to or less than about 200 μm, or equal to or less than about 150 μm, or equal to or less than about 125 μm, or equal to or less than about 100 μm, or equal to or less than about 90 μm, or equal to or less than about 80 μm, or equal to or less than about 70 μm, or equal to or less than about 60 μm, or equal to or less than about 50 μm, or equal to or less than about 40 μm, or equal to or less than about 30 μm, or equal to or less than about 20 μm, or equal to or less than about 10 μm.

The fibrous substrate comprising cellulose may be microfibrillated to obtain microfibrillated cellulose having a modal fibre particle size ranging from about 0.1-500 μm. The fibrous substrate comprising cellulose may be microfibrillated in the presence to obtain microfibrillated cellulose having a modal fibre particle size of at least about 0.5 μm, for example at least about 10 μm, or at least about 50 μm, or at least about 100 μm, or at least about 150 μm, or at least about 200 μm, or at least about 300 μm, or at least about 400 μm.

The fibrous substrate comprising cellulose may be microfibrillated in the presence of an inorganic particulate material to obtain microfibrillated cellulose having a fibre steepness equal to or greater than about 10, as measured by Malvern. Fibre steepness (i.e., the steepness of the particle size distribution of the fibres) is determined by the following formula:

Steepness=100×(d ₃₀ /d ₇₀)

The microfibrillated cellulose may have a fibre steepness equal to or less than about 100. The microfibrillated cellulose may have a fibre steepness equal to or less than about 75, or equal to or less than about 50, or equal to or less than about 40, or equal to or less than about 30. The microfibrillated cellulose may have a fibre steepness from about 20 to about 50, or from about 25 to about 40, or from about 25 to about 35, or from about 30 to about 40.

The particle size distribution may be calculated from Mie theory and gave the output as a differential volume based distribution. The presence of two distinct peaks was interpreted as arising from the mineral (finer peak) and fibre (coarser peak).

The finer mineral peak was fitted to the measured data points and subtracted mathematically from the distribution to leave the fibre peak, which can be converted to a cumulative distribution. Similarly, the fibre peak can be subtracted mathematically from the original distribution to leave the mineral peak, which x can also be converted to a cumulative distribution. Both these cumulative curves can then be used to calculate the mean particle size (d₅₀) and the steepness of the distribution (d₃₀/d₇₀×100). The differential curve may be used to find the modal particle size for both the mineral and fibre fractions.

In one embodiment, the grinding vessel is a tower mill. In another embodiment, the grinding is performed in a screened grinder, preferably a stirred media detritor. The screened grinder may comprise one or more screen(s) having a nominal aperture size of at least about 250 μm, for example, the one or more screens may have a nominal aperture size of at least about 300 μm, or at least about 350 μm, or at least about 400μm, or at least about 450 μm, or at least about 500μm, or at least about 550 μm, or at least about 600 μm, or at least about 650 μm, or at least about 700 μm, or at least about 750 μm, or at least about 800 μm, or at least about 850 μm, or at or least about 900 μm, or at least about 1000 μm or at least 1250 μm or 1500 μm. The screen sizes noted immediately above are applicable to use of a tower mill embodiments as well.

As noted above, the grinding is performed in the presence of a grinding medium. In an embodiment, the grinding medium is a coarse media comprising particles having an average diameter in the range of from about 1 mm to about 6 mm, for example about 2 mm, or about 3 mm, or about 4 mm, or about 5 mm.

In another embodiment, the grinding media has a specific gravity of at least about 2.5, for example, at least about 3, or at least about 3.5, or at least about 4.0, or at least about 4.5, or least about 5.0, or at least about 5.5, or at least about 6.0.

As described above, the grinding medium (or media) may be in an amount up to about 70% by volume of the charge. The grinding media may be present in amount of at least about 10% by volume of the charge, for example, at least about 20% by volume of the charge, or at least about 30% by volume of the charge, or at least about 40% by volume of the charge, or at least about 50% by volume of the charge, or at least about 60% by volume of the charge.

In one embodiment, the grinding medium is present in amount of about 50% by volume of the charge. By charge' is meant the composition which is the feed fed to the grinder vessel. The charge includes water, grinding media, the fibrous substrate comprising cellulose and any other optional additives (other than as described herein). The use of a relatively coarse and/or dense media has the advantage of improved (i.e., faster) sediment rates and reduced media carry over through the quiescent zone and/or classifier and/or screen(s). A further advantage in using relatively coarse screens is that a relatively coarse or dense grinding media can be used in the microfibrillating step. In addition, the use of relatively coarse screens (i.e., having a nominal aperture of least about 250 μm) allows a relatively high solids product to be processed and removed from the grinder, which allows a relatively high solids feed (comprising fibrous substrate comprising cellulose and inorganic particulate material) to be processed in an economically viable process. As discussed below, it has been found that a feed having a high initial solids content is desirable in terms of energy efficiency. Further, it has also been found that product produced (at a given energy) at lower solids has a coarser particle size distribution.

In accordance with one embodiment, the fibrous substrate comprising cellulose is present in the aqueous environment at an initial solids content of at least about 1 wt. %. The fibrous substrate comprising cellulose may be present in the aqueous environment at an initial solids content of at least about 0.25%, or at least about 0.5%, or at least about 1.5%, or at least about 2 wt. %, or for example at least about 3 wt. %, or at least about at least 4 wt. %. Typically, the initial solids content will be no more than about 10 wt. %.

In another embodiment, the grinding is performed in a cascade of grinding vessels, one or more of which may comprise one or more grinding zones. For example, the fibrous substrate comprising cellulose may be ground in a cascade of two or more grinding vessels, for example, a cascade of three or more grinding vessels, or a cascade of four or more grinding vessels, or a cascade of five or more grinding vessels, or a cascade of six or more grinding vessels, or a cascade of seven or more grinding vessels, or a cascade of eight or more grinding vessels, or a cascade of nine or more grinding vessels in series, or a cascade comprising up to ten grinding vessels. The cascade of grinding vessels may be operatively inked in series or parallel or a combination of series and parallel. The output from and/or the input to one or more of the grinding vessels in the cascade may be subjected to one or more screening steps and/or one or more classification steps.

The total energy expended in a microfibrillation process may be apportioned equally across each of the grinding vessels in the cascade. Alternatively, the energy input may vary between some or all of the grinding vessels in the cascade.

A person skilled in the art will understand that the energy expended per vessel may vary between vessels in the cascade depending on the amount of fibrous substrate being microfibrillated in each vessel, and optionally the speed of grind in each vessel, the duration of grind in each vessel and the type of grinding media in each vessel. The grinding conditions may be varied in each vessel in the cascade in order to control the particle size distribution of the microfibrillated cellulose.

In an embodiment the grinding is performed in a closed circuit. In another embodiment, the grinding is performed in an open circuit.

As the suspension of material to be ground may be of a relatively high viscosity, a suitable dispersing agent may preferably be added to the suspension prior to grinding. The dispersing agent may be, for example, a water soluble condensed phosphate, polysilicic add or a salt thereof, or a polyelectrolyte, for example a water soluble salt of a poly(acrylic add) or of a poly(methacrylic add) having a number average molecular weight not greater than 80,000. The amount of the dispersing agent used would generally be in the range of from 0.1 to 2.0% by weight, based on the weight of the dry inorganic particulate solid material. The suspension may suitably be ground at a temperature in the range of from 4° C. to 100° C.

Other additives which may be included during the microfibrillation step include: carboxymethyl cellulose, amphoteric carboxymethyl cellulose, oxidising agents, 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO), TEMPO derivatives, and wood degrading enzymes.

The pH of the suspension of material to be ground may be about 7 or greater than about 7 (i.e., basic), for example, the pH of the suspension may be about 8, or about 9, or about 10, or about 11. The pH of the suspension of material to be ground may be less than about 7 (i.e., acidic), for example, the pH of the suspension may be about 6, or about 5, or about 4, or about 3. The pH of the suspension of material to be ground may be adjusted by addition of an appropriate amount of acid or base. Suitable bases included alkali metal hydroxides, such as, for example NaOH. Other suitable bases are sodium carbonate and ammonia. Suitable acids included inorganic acids, such as hydrochloric and sulphuric acid, or organic acids. An exemplary acid is orthophosphoric acid.

The total energy input in a typical grinding process to obtain the desired aqueous suspension composition may typically be between about 100 and 1500 kWht⁻¹ based on the total dry weight of the inorganic particulate filler. The total energy input may be less than about 1000 kWht⁻¹, for example, less than about 800 kWht⁻¹, less than about 600 kWht⁻¹, less than about 500 kWht⁻¹, less than about 400 kWht⁻¹, less than about 300 kWht⁻¹, or less than about 200 kWht⁻¹.

A cellulose pulp can be microfibrillated at relatively low energy input when it is co-ground in the presence of an inorganic particulate material. The total energy input per tonne of dry fibre in the fibrous substrate comprising cellulose will be less than about 10,000 kWht⁻¹, for example, less than about 9000 kWht⁻¹, or less than about 8000 kWht⁻¹, or less than about 7000 kWht⁻¹, or less than about 6000 kWht⁻¹, or less than about 5000 kWht⁻¹, for example less than about 4000 kWht⁻¹, less than about 3000 kWht⁻¹, less than about 2000 kWht⁻¹, less than about 1500 kWht⁻¹, less than about 1200 kWht⁻¹, less than about 1000 kWht⁻¹, or less than about 800 kWht⁻¹. The total energy input varies depending on the amount of dry fibre in the fibrous substrate being microfibrillated, and optionally the speed of grind and the duration of grind.

Embodiments of the present invention will now be described by way of illustration only, with reference to the following examples.

Low Solids Compositions of MFC

The present inventors have also determined that production of MFC utilizing low solids levels in the microfibrillation process has a beneficial impact on MFC produced by the grinding processes described in this specification. The concentration of fibres in the grinder is directly proportional to the residence time at a given specific energy input and intensity and is the parameter that most strongly influences charge rheology. MFC production reported in Example 2 has shown that there is a relatively strong influence of the fibre solids on the tensile index of the resultant MFC, when other operating conditions and specific energy input are maintained as constant. Pilot scale work has shown excellent results, for example, using Birch fibre at 0.75% fibre solids. The work shown in Example 2 and FIG. 13A has demonstrated that the optimum specific energy input is only weakly dependent on fibre solids across the range tested, although it is known that below a certain solids content value energy efficiency rapidly decreases (e.g. see FIG. 7).

The combination of the method of predicting desired tensile indices of cellulose substrates in combination with the grinding processes described in this specification at low solids levels yields MFC with marked improvements in tensile index.

EXAMPLES

Example 1: Investigation of hemicellulose content and zero-span tensile index of cellulose feedstock, and the zero-span tensile index of nanopapers manufactured from various cellulose feedstocks. Nanopaper is herein defined as a composite sheet of cellulose and inorganic mineral wherein the cellulose fraction is dominated by microfibrillated cellulose rather than intact fibres.

Material and Methods

Fibre Sources

In total, twenty-four separate fibre species were investigated, from a variety of wood and non-wood sources. These are listed in Table 1 below.

TABLE 1 Fibre Sources Table 1 - Fibre species used in this study. Softwoods Hardwoods Non-woods Nordic Pine Birch #1 Cotton (linters) Black Spruce Birch #2 Jeans‡ Radiata Pine Eucalyptus Abaca Southern Pine Acacia Sisal Douglas Fir Mixed South Kenaf Asian Hardwood Enzyme-treated Mixed European Giant Reed Nordic Pine Hardwood (Arundo Donax) Dissolving Pulp* Tissue Dust† Bagasse (sugar cane) Miscanthus Sorghum Flax *Dissolving Pulp is a softwood that was subjected to the sulphite bleaching process, rather than the kraft (sulphate) process which all other softwoods and hardwoods listed underwent. †Waste short fibres collected from the dust extraction system of a tissue mill. ‡Cotton fibres from recycled jeans, following disintegration into individual fibres and formation into a pulp board.

Pulp Preparation

Fibres were added to the grinding process as a 30% solids cake. Miscanthus, sorghum, and giant reed were supplied in this form, whereas most other fibre sources were supplied as dry pulp boards. These dry pulps were soaked in water and broken down into individual fibres using a pulp disintegrator for 10 minutes. Excess water was removed by a vacuum filter, forming a cake. To avoid choking of the grinder due to entanglement of very long fibres, the flax fibres were cut with a guillotine into fragments around 3 mm in length.

MFC Production by Stirred Media Detritor Grinding

MFC was produced in a grinder that uses the motion of media beads to disintegrate the fibres. The presence of micron-scale mineral particles greatly improves the efficiency of this process, so the fibre charge was complemented with an equal amount (on a dry mass basis) of IC60 ground calcium carbonate mineral (Imerys). Sufficient water was also added to form a slurry with the target fibre solids content. When a grind was completed, the slurry was separated from the media with a vibrating screen, or by a washing method when particles were too coarse for the screen.

Previous experience with this process has shown that almost all fibre sources reach a peak tensile index at around the same energy. Thus, a single energy input of 3,000 kWh/t was chosen at which MFC was produced using each of the fibre species listed in Table 1.

Grinds were carried out at 800 RPM, 50% POP (percentage of pulp), 47.5% MVC and 3000 kWh/t, over a range of solids contents. In this example, 1411 g of 3 mm mullite media with a specific gravity of 2.7 g/cm³ was used as the grinding media, and the mineral used was ground calcium carbonate (Imerys IC60). This grinder is a vertical mill comprising a cylindrical grinding vessel having an internal diameter of 14.5 cm and a vertical impeller shaft having a circular cross section and a diameter 1.8 cm. The shaft is equipped with 4 impellers positioned in an X design. The impellers have a circular cross section and a diameter 1.8 cm. The impellers are 6.5 cm long measured from the centre of the vertical shaft to the impeller tip

Nanopaper Sheet Preparation

The MFC-mineral composite product was collected as an aqueous slurry. The solids content was measured by weighing a sample before and after oven drying. The mineral content was determined from the change in weight of an oven-dried sample after burning off the cellulose in a 450° C. furnace for 2 hours. Using this information, sufficient IC60 calcium carbonate mineral was added to the sample to dilute the WC content to 20 wt % on a dry mass basis. These samples were used to form a nanopaper sheet in a vacuum sheet former and dried using a Rapid-Kothen dryer, targeting a weight of 220 gsm. These sheets were conditioned in a controlled environment at 23° C. and 50% relative humidity overnight prior to tensile testing.

Nanopaper Tensile Testing

The nanopaper samples were weighed and cut into 15 mm width strips. These strips were clamped in a Testometric M350 tensile tester and strained at a constant rate until failure. The force applied at break was divided by the width and gsm to obtain the tensile index in Nm/g. The tensile index of this WC-mineral composite nanopaper is many times weaker than pure WC due to the mineral disrupting inter-fibril bonding. However, work with this process has shown that the tensile indices of WC with or without a given fraction of mineral added are proportional. Phipps, J., Larson, T., Ingle, D., Eaton, H., 2017, “The Effect of Microfibrillated Cellulose on the Strength and Light Scattering of Highly Filled Papers,” In: Batchelor, W., Söderberg, D., 2017. Advances in Pulp and Paper Research. Transactions of the 16th Fundamental Research Symposium, September 2017, Oxford, UK, pp.231-254.Three nanopaper sheets were produced for each sample, each cut into five strips. The mean average tensile index of these fifteen strips is reported as the sample tensile index here.

Fibre Image Analysis

A Valmet FS5 fibre image analyser was used to measure geometric parameters of the original fibres before grinding, and the MFC particles after grinding. The Valmet FS5 Fibre Image Analyser (from here onwards referred to as the “fibre analyser”) is a machine that is used to assess dimensions of fibres, such as length and width distributions. In such a test, a roughly 500 mL suspension of roughly 0.002 wt % fibre solids is produced, and loaded into the machine. The machine mixes the suspension, and pumps it through some transparent tubing past a camera. The camera takes thousands of pictures, and the fibre analyser software uses various algorithms in order to determine fibre dimensions and other geometric and morphological properties.

The fibre analyser is conventionally used to measure unrefined or refined cellulose fibres, but it is evident that it is capable of inferring such geometric properties of the coarser components of microfibrillated cellulose also, for which it has proven useful for the subject of this patent. Key geometric properties that the fibre analyser measures include:

Fibre length—this is defined as a mean fibre length. The machine produces six different mean fibre lengths, with the mean weighted in a different way for each. For the purpose of this invention, the length-weighted fibre length is used as defined by the TAPPI T 271 om-12 standard. ‘Fibre length’ does not exactly mean the same thing when cellulose has been fully disintegrated into MFC, since there are very few intact fibre segments remaining; instead, in the case of MFC, fibre length represents the largest dimension of the MFC particles, which appear to be webs of entangled or physically connected loose fibrils. The machine outputs this length as a “fibre length”, which is what is being referred to when MFC fibre length is being discussed, but more accurately this represents the longest dimension of the MFC particles, since few intact fibres remain.

Fibre width—this the length-weighted mean fibre width, which is measured as the mean thickness of the fibres in the direction perpendicular to their lengths. For fully processed MFC, fibre width is defined as the width of the particle in the direction perpendicular to the direction for which the MFC fibre length is defined.

Kink—Calculated from the Kibblewhite kink index, it is a measurement of the number of sharp changes in direction of the fibre length per metre of fibre length. With MFC, the kink is similarly a measure of the frequency of sharp changes in direction along the length (largest dimension) of the MFC particle.

Fines B—The percentage by total length of the sample that are categorised as ‘high aspect ratio fines’ by the software, defined as particles with widths lower than 10 μm and lengths longer than 0.2 mm. These are believed to be loose fibrils or fibril bundles.

Fibrillation %—Mean degree of ‘external fibrillation’ of the fibres, in other words fibrils that are still attached to the main fibre structure, that have not been broken off loose in suspension. The fibre analyser estimates this based upon the colour contrast gradient of the border of the fibrous particle; it assumes that the presence of a broad gradient is due to the presence of extended fibrils below the measurement limit of the camera contributing some greyness and thereby reducing the contrast between the fibre and fluid. The greater the number of these extended fibrils, and the longer their lengths, the less sharp that this colour contrast would be.

A sample suspension is pumped past a camera that takes thousands of images, and parameters such as length and width distributions are determined by a computer algorithm. Though designed for fibre measurement, this equipment is sensitive enough to measure MFC particles with lengths around a hundred microns and above.

Hemicellulose Content Measurements

Predicting the MFC tensile index properties from easily measurable properties such as hemicellulose content (mass fraction) and zero-span tensile index measurements (Nm/g) enable selection of fibrous substrates comprising cellulose to produce MFC with increased tensile index properties without the necessity of fibrillating each feed stock to determine such properties.

Hemicellulose measurements were performed by Labtium Oy, who used the SCAN-CM 71:09 method. Scandinavian Pulp, Paper and Board Testing Committee, 2009. Carbohydrate Composition. Test Method SCAN CM-71:09. Fibre samples underwent sulphuric acid hydrolysis, and the resultant concentration of various monomer sugar residues were detected by chromatography. This included glucose, xylose, mannose, arabinose, and galactose. To convert these sugar contents into xylan and glucomannan hemicellulose contents, it was assumed that the xylan content was the sum of all the xylose and arabinose, and the glucomannan content consisted of all the galactose, mannose, and 1 unit of glucose for every 4 units of mannose, since a ratio of 1:4 glucose to mannose is typical in softwood glucomannan. Ebringerová, A., 2006. Total hemicellulose content was determined by adding together the xylan and glucomannan fractions.

Fibre Zero-Span Tensile Testing

Zero-span tensile tests were carried out on the initial fibre samples rather than the MFC. The TAPPI Standard T 231 cm-07 was followed, using 60 gsm mineral-free handsheets for each fibre species produced according to TAPPI standard T 205 sp-12. Technical Association of the Pulp and Paper Industry, 2007. Zero-span breaking strength of pulp (dry zero-span tensile). Test Method T 231 cm-07; Technical Association of the Pulp and Paper Industry, 2007. Forming Handsheets for Physical Tests of Pulp. Test Method T 205 cm-12. Each handsheet was cut into five strips, which were clamped in a Pulmac zero-span tensile tester and strained until broken. As with conventional tensile testing, the force measured was converted to a zero-span tensile index. Four dry sheets were tested for each fibre species, and a mean average result is reported herein.

Scanning Electron Microscopy

A Jeol 6700 Scanning Electron Microscope was used to image several MFC samples produced at the standard energy input, at ×10000 magnification. Preparation involved filtering a dilute MFC sample through a 0.2 μm nucleopore membrane. The samples were sputter coated with platinum to form a conductive monolayer. Secondary electrons emitted from the sample during measurement were used to form the images. Four images were taken for each sample.

Results

Influence of Hemicellulose Content

In bleached chemical pulp, hemicellulose is found within the fibre cell wall. The hemicellulose molecules form a layer around the cellulose microfibrils which separates adjacent microfibrils. The hemicellulose content for each fibre species was measured, and is plotted against the MFC tensile index as FIG. 2. The fibrous substrates comprising cellulose tested in Example 1 include Nordic Pine, Black Spruce, Radiata Pine, Southern Pine, Enzyme-Treated Nordic Pine, Douglas Fir, Dissolving Pulp, Birch #1, Birch #2, Eucalyptus, Acacia, Mixed European Hardwood, Mixed South Asian Hardwood, Tissue Dust, Cotton, Jeans, Abaca, Sisal, Bagasse, Kenaf, Miscanthus, Sorghum, Giant Reed and Flax.

FIG. 2 shows that a high hemicellulose content of the fibre correlates with a high MFC tensile index. The R² value of a linear correlation is around 0.63, which is moderately good for a single parameter, considering that cellulose is a natural product with a complex hierarchy of structures with different scales and chemistries between plant species.

As discussed in the introduction and without being bound by theory, there are two mechanisms by which hemicellulose is thought to improve MFC tensile index. One mechanism is that a high hemicellulose fibre when disintegrated to form MFC has more of this liberated surface area coated in hemicellulose, which is thought to form bridges between particles upon drying and thereby improve relative bonded area.

The other mechanism is that the presence of hemicellulose, being a relatively mechanically weak amorphous layer, provides a preferred plane of breakage along the microfibril lengths, thereby facilitating disintegration into finer microfibrils, increasing the aspect ratio. To assess whether the latter mechanism is taking place, SEM images were taken of samples with a range of hemicellulose contents to assess fibril and microfibril diameters. Four of such images are displayed as FIG. 3A-D.

FIG. 3A shows Bagasse MFC, which had the highest hemicellulose content of any fibre tested, at 28%. There appears to be a much higher frequency of finer microfibrils in Bagasse than Dissolving Pulp and Cotton MFC, shown as FIG. 3C and FIG. 3D, which have 4% and 0% hemicellulose respectively. Nordic Pine MFC with a hemicellulose content of 17% is displayed as FIG. 3B, and also shows a high abundance of fine microfibrils, though to a lesser extent than Bagasse MFC.

A comparison of FIG. 3B and FIG. 3C is noteworthy. The Dissolving Pulp used in the FIG. 3C is a Scandinavian pine/spruce blend, similar to the Nordic Pine used in FIG. 3B. However, the key difference is in their respective processing. The Nordic Pine pulp in FIG. 3B underwent a kraft pulping process, which cannot penetrate well into the fibres to remove hemicellulose. The Dissolving Pulp of FIG. 3C was pulped using a sulphite process, which is capable of removing the majority of the hemicellulose from deep within the fibre cell wall. The liberated microfibrils in the Nordic Pine MFC of FIG. 3B tend to be much finer than with the Dissolving Pulp MFC of FIG. 3C. This difference implies that the correlation of hemicellulose content with finer microfibrils is causal. The contribution of the improved bonding due to hemicellulose being present on the microfibril surface is unknown, but the differences in microfibril width at different hemicellulose contents suggests that this surface effect does not dominate.

Influence of Feed Fibre Geometry

Fibre geometry has a strong influence in tensile strength in paper made from cellulose fibres. Longer and thinner fibres result in higher tensile strengths because they maximise aspect ratio. Longer fibres increase the number of connections that each individual fibre can make, and thereby distribute stress over a larger area. Finer fibres (with a thinner cell wall) result in more fibres per unit mass. The fact that a high fibre aspect ratio leads to high sheet tensile strength has been found experimentally for both softwoods and hardwoods Horn, R. A., 1974, “Morphology of pulp fiber from softwoods and influence on paper strength,” Research Paper FPL 242, Forest Product Laboratory, Forest Service, United States Department of Agriculture; Horn, R. A., 1978. Morphology of pulp fiber from hardwoods and influence on paper strength. Research Paper FPL 312, Forest Product Laboratory, Forest Service, United States Department of Agriculture. The Page Equation, shown as Equation [1], makes use of such fibre dimensions to model paper tensile index.

Geometric parameters of the fibres were measured using a fibre image analyser, including length and length distributions, widths, and coarseness (a measure of fibre wall cross sectional area). Attempts were made to plot such geometric parameters against tensile index, and a clear correlation was not apparent. The geometry of the feed fibres is therefore not thought to have a strong correlation with the quality of fully processed MFC.

Influence of MFC Length

Stirred media detritor grinding, as described elsewhere in this specification, produces MFC mostly in the form of fibril aggregates rather than fully individualised fibrils, i.e. the fibrils are liberated from the fibre, but tend to be physically rooted to other fibrils to form a network. These aggregates are large enough to have some geometric properties inferred from measurement in the fibre analyser. In this study, the length-weighted fibre length of these MFC aggregates, designated herein as “MFC length” was found to be useful. The fibre analyser interprets the ‘fibre length’ in this case as the longest dimension of an MFC aggregate particle. Although not presented here, MFC length was plotted against MFC tensile index, and no general correlation was found. However, points with similar hemicellulose contents appear to cluster in regions within which there seemed to be a correlation. Therefore, the combination of hemicellulose and MFC length to predict tensile index was believed to correlate.

The Page Equation was used as a starting point to rationalise the correlation of hemicellulose content and fibre zero-span tensile index to MFC tensile index. The Page Equation, shown as Equation [1], was formulated to predict tensile index for straight, individualised fibres with lengths on the scale of millimetres, when formed into a mineral-free sheet. Thus, the Page Equation is conceptually useful, since parameters such as fibre length and relative bonded area are expected to be applicable in a similar way to this MFC-inorganic particulate material composite nanopaper. Accordingly, the Page Equation was used to arrive at a semi-empirical model to predict MFC tensile index using hemicellulose content and MFC length.

In considering the Page Equation, as relative bonded area increases greatly with MFC production, the bonding term becomes less limiting. Strength, therefore, would be expected to be eventually dominated by the zero-span term representing fibre or fibril breakage. In a published study attempting to measure zero-span tensile index of MFC, it was found that the sample failed mostly due to network bonding failure, not individual microfibril failure, implying that even with MFC, tensile index is still controlled by the bonding term in the Page Equation rather than the fibre strength term. Varanasi, S., Chiam, H. H., Batchelor, W., 2012, “Application and interpretation of zero and short-span testing on nanofibre sheet materials,” Nordic Pulp and Paper Research Journal, 27(2): 343-506.

The MFC used in the present study was tested with 80% filler mineral, which greatly reduces inter-fibre bonding compared to an unfilled sheet. Therefore, in the present case it is a reasonable assumption that the MFC sheet will fail due to inter-particle bonding rather than failures in the fibril or microfibril cross-sections. Consequently, the fibre weakness term will be assumed to be zero as the MFC zero-span tensile index is very high compared to bonding strength. The Page Equation is therefore reduced to:

$\begin{matrix} {\frac{1}{T} = \frac{12A\rho}{\tau_{B}P{L({RBA})}}} & \lbrack 2\rbrack \end{matrix}$

Hemicellulose on microfibril surfaces would be expected to increase relative bonded area since extended hemicellulose chains allow for more extensive and intimate contact between microfibrils. Additionally, hemicellulose appears to result in finer microfibrils when MFC is produced. Finer diameter microfibrils could be expected to be more flexible, and so be more capable of deforming for a more intimate contact with other particles. It is also expected that finer microfibrils would be more susceptible to capillary forces drawing them in contact with other surfaces during drying. These effects would imply that a high hemicellulose content would lead to a high relative bonded area.

The perimeter to cross-sectional area ratio, “P/A”, would also correlate with increased bonding; since P/A increases as fibril diameter decreases, P/A should also correlate with hemicellulose content. Though both P/A and RBA are believed to be influenced by hemicellulose content, their relative influences cannot be readily distinguished and quantified. If hemicellulose correlated linearly with both, then it would be expected that tensile index would be proportional to the square of the hemicellulose content; however, FIG. 2 shows that this correlation is linear. Rather than distinguishing between the two, both the RBA term and the P/A term in the denominator of the Page Equation are replaced with the hemicellulose content H in Equation [3] below. Since the larger fibril aggregates are the primary load-bearing particles, the corresponding MFC length term L_(MFC) is expected to be analogous to the fibre length term L in the Page Equation, and so this substitution is also made:

$\begin{matrix} {\frac{1}{T} = {k\frac{12\rho}{\tau_{B}L_{MFC}H}}} & \lbrack 3\rbrack \end{matrix}$

where k is a proportionality constant. Density p is constant for all MFC, and the specific bond strength τ_(B)is not expected to change significantly since the chemistry of the bonded surface would not vary much between MFC samples. The Page Equation can therefore be rewritten as follows, with the constant terms collected under a single coefficient B₁:

$\begin{matrix} {\frac{1}{T} = \frac{1}{B_{1}L_{MFC}H}} & \lbrack 4\rbrack \end{matrix}$

Equation [4] can be inverted to obtain the MFC tensile index in Equation [5]:

T=B₁L_(MFC)H   [5]

FIG. 4 gives σ₀ a value of 4.1 Nm/g on a basis of the fit with the twenty-four fibre sources tested. σ₀ is added to Equation [5] to provide Equation [6]:

T=B ₁ L _(MFC) H+σ ₀   [6]

Although the fit is good, using the MFC length in order to identify which fibre sources are worth using as a feed source is impractical; the MFC must first be created to get this length value, and so this only reduces the need of the tensile test, saving little effort overall. Instead, it is more useful to identify the reasons behind why different fibres give MFC of different MFC aggregate lengths, and relate this to a parameter that can be measured in the unprocessed fibres.

Influence of Fibre Zero-Span Tensile Index

It stands to reason that if the fibrils that form the cell wall are long, that once the fibre is disintegrated the liberated fibrils will also be long. Additionally, a greater number of flaws in the fibril structure would make breaking the fibril (and the larger scale fibre) across the cross-section easier. Therefore, a fibre that has intrinsically long fibrils with few discontinuities, with these fibrils having a low frequency of flaws within their cross section, would be expected to produce long fibrils when ground down into MFC, and would therefore give a relatively high MFC length as measured by the fibre analyser. As discussed in the introduction, the zero-span tensile test gives an indication of the frequency of these flaws by forcing fibres to break across their cross section, with a high value indicating a lack of flaws and discontinuities.

FIG. 5 shows the correlation between the fibre zero-span tensile index and the fibre length of the resultant MFC. There appears to be a linear relation between these two parameters when considering most fibre species, although there are several exceptions that deviate significantly, including flax, sisal, and kenaf. The reasons for this are unknown, but it is suspected that differences in microfibril angle (the angle of the helix that the fibrils form when they are entwined in the fibre cell wall) would have an influence.

The zero-span tensile test is not solely a measure of fibre and fibril damage. A good correlation has been found between the microfibril angle and the zero-span tensile index, at least with fibres that are relatively undamaged by processing. Courchene, C. E., Peter, G. F., Litvay, J., 2006, “Cellulose microfibril angle as a determinant of paper strength and hygroexpansivity in Pinus Taeda L,” Wood and Fiber Science, 38(1): 112-120. This agrees with data and theoretical modelling from El-Hosseiny, F. and Page, D. H., 1975. “The mechanical properties of single wood pulp fibres: Theories of strength,” Fibre Science and Technology, 8(1): 21-31. However, the microfibril angle is not expected to directly influence fibril lengths or frequency of fibril damage within the fibre, and it would therefore be surprising if it influences fibril length in MFC. Although easy to measure in wood, the microfibril angle is difficult to measure accurately in pulped fibres, and so was not attempted in Example 1 because it is impractical in an industrial setting.

Despite this complication, the fit between MFC length and zero-span tensile index for most fibre species appears sufficient for most of the other fibre species to use zero-span tensile index as a replacement for MFC length in Equation [6]. This equation is therefore modified below:

T=B ₂ ZH+σ ₀   [7]

where Z represents the zero-span tensile index of the fibre and B₂ is a proportionality coefficient. FIG. 5 shows the correlation between the product of the fibre zero-span tensile index and hemicellulose content, with the MFC tensile index.

As FIG. 5 shows, the assumption that fibre zero-span tensile index correlates with MFC length is not completely accurate, so it is not surprising that the fit shown in FIG. 6 for Equation [3] (R²=0.78) is significantly worse than when MFC length was used (R²=0.87). Although this relationship fits well considering the extremes, towards the centre of the graph certain fibre species such as Miscanthus, Abaca, and Sisal deviate considerably from the best fit curve. For most fibre species, however, the fit is good, and this relationship comes with the important advantage that both predictive parameters are fibre properties that can be measured relatively easily without having to produce the MFC first. This relationship would be a practical tool for shortlisting a large number of fibre species, in order to determine which are worth pursuing to use as a feed for MFC production.

Using the Page Equation [7] in modified form provides that measurements of the hemicellulose content and zero-span tensile index of pulp fibres can be used for a reasonably accurate prediction of the resultant MFC tensile index, thereby providing a useful method to aid in the selection of cellulose sources for use as a feedstock for microfibrillated cellulose production.

FIG. 12 is a plot of predicted tensile index versus measured tensile index of MFC produced in Example 1 from 24 diverse sources of fibrous substrates comprising cellulose.

The fibre analyser measures a ‘fibre length’ of MFC, representing the largest dimension of the fibrillated bundles.

Ideal cellulose feed stocks of cellulose fibres are shown in the various Figures of this specification.

Example 2: Investigation of impact of solids content on tensile index of MFC produced by various cellulose feed stocks.

Using a grinding process as described in this specification, we determined that energy sweeps of 3,000 kWh/t performed at a 2.5% fibre solids standard to be the specific energy at which all fibre species produce MFC close to their maximum strength. We therefore decided to maintain specific energy of 3,000 kWh/t for all the experiments in this Example 2, and to vary the fibre solids content. Necessarily, this results in a change in absolute energy input.

Grinds were carried out at 800 RPM, 50% POP, 47.5% MVC and 3000 kWh/t, over a range of solids contents. Since these grinds took place at 50% POP, the fibre solids content in this example is found by halving the total solids content.

Solids content can influence viscosity, and therefore flow regimes and the mobility of a fibre in response to shear. Additionally, residence time is increased with increasing fibre solids since the absolute energy scales with solids content to match the same specific energy. For pulps that respond well to low solids grinding, viscosity increases with lower solids to a greater proportion than tensile index.

Several pulps were investigated an included: Nordic Pine, Birch, Eucalyptus. Acacia and Enzyme-Treated Nordic Pine.

In general, lowering the solids content improves product quality. Higher quality pulps benefit more from low solids grinding. The peak in strength occurs at different places depending on pulp type. We determined that the peak strength occurred at the following total solids contents in: Nordic Pine˜1.5% solids; Birch˜2% solids and Acacia˜4% solids. The data is plotted in FIG. 7.

FIG. 13A is a graph of energy input versus tensile index of aqueous slurries of MFC produced from Birch fibres and inorganic particulate material at solids contents of 1.5% total solids (0.75 fibre solids), 3% total solids (1.5% fibre solids) and 5% total solids (2.5% fibre solids).

As shown in FIG. 13A, lowering the grinding solids increases the MFC tensile index considerably. Optimum energy input appears to increase slightly, but not by as much as expected. FIG. 13B shows that tear strength is also higher when grinding solids are lower, implying larger individual load-bearing particles.

FIGS. 14A and 14B are graphs of energy input (kWh/t) versus fibre length (in mm) and fibre width (in μm) as measured by the Valmet FS5 Fibre Image Analyser. FIG. 14A plots of energy input (kWh/t) versus fibre length (in mm) for MFC produced from aqueous slurries of Birch fibres and inorganic particulate material at solids contents of 1.5% total solids (0.75 fibre solids), 3% total solids (1.5% fibre solids) and 5% total solids (2.5% fibre solids). FIG. 14B plots energy input (kWh/t) versus fibre width (in mm) for MFC produced from aqueous slurries of Birch fibres and inorganic particulate material at solids contents of 1.5% total solids (0.75 fibre solids), 3% total solids (1.5% fibre solids) and 5% total solids (2.5% fibre solids). The graphs demonstrate an exponential decay of fibre length with energy in a similar manner to fibrous particle d₅₀ measurements by laser light scattering.

FIG. 15A is a graph of energy input (kWh/t) versus the percentage of high aspect ratio fines detected in the fibrillation process of Example 2 as measured by the Valmet FS5 Fibre Image Analyser. FIG. 15B is graph of energy input (kWh/t) versus fibrillation percentage as measured by the Valmet FS5 Fibre Image Analyser for MFC produced from aqueous slurries of Birch fibres and inorganic particulate material at solids contents of 1.5% total solids (0.75 fibre solids), 3% total solids (1.5% fibre solids) and 5% total solids (2.5% fibre solids). It is evident that a very high degree of fibrillation occurred in the lowest total solids grinds. High aspect ratio fines were higher at higher total solids contents of the aqueous suspensions. This is expected because external fibrils are broken off and separated as fines at the higher absolute energy inputs experienced by the higher solids grinds.

Example 3. Investigation of low solids grinds of an expanded group of fibrous substrates comprising cellulose.

The experimental procedure of Example 2 is repeated in this Example 3 utilizing fibrous substrates comprising cellulose of Abaca, Cotton Linters, Jeans Cotton, Kenaf, Bagasse (sugar cane), Tissue Dust and Mixed European hardwood. The fibrous substrates were selected to determine why certain substrates benefit from low solids grinding, whereas it is detrimental to others.

FIG. 19A is a graph of total solids content of the aqueous suspensions comprising different fibrous substrates comprising cellulose and inorganic particulate material versus tensile index (Nm/g) determined for the microfibrillated cellulose produced from the different fibrous substrates comprising cellulose according to Example 3. FIG. 19B is a graph of total solids content of the aqueous suspensions comprising different fibrous substrates comprising cellulose and inorganic particulate material versus viscosity at 10 rpm (mPas) determined for the microfibrillated cellulose produced from the different fibrous substrates comprising cellulose according to Example 3.

FIG. 22 is a graph of MFC tensile index at 5% total solids (2.5% fibre solids) ground according to Example 3 plotted against the difference in tensile index between MFC produced with the same fibre source at 2% total solids and that produced at 8% total solids (a measurement of the gradient of the solids content vs. MFC tensile index curve on FIG. 19A) for fibrous substrates comprising cellulose of Abaca, Cotton, Jeans, Kenaf, Bagasse, Tissue Dust, Mixed European Hardwood, Nordic Pine, Birch, Acacia, Enzyme-Treated Nordic Pine and Eucalyptus. It can be seen that the gradient of the solids content vs. tensile index curve correlates well with the MFC tensile index at ‘standard’ conditions of 5% total solids (or 2.5% fibre solids). Better quality pulps respond better to lower solids fibrillation by grinding. This is likely to be due to the grinding conditions grinding the fibrous substrate comprising cellulose to a lesser degree resulting in preservation of long liberated fibrils. For high quality MFC having increased tensile index values, there are numerous long, liberated fibrils. For lower quality MFC having lower tensile index values, the fibrils are shorter and the fibrillation conditions are not improved by low solids grinding processes. In such a case, the under-grinding effect predominates resulting in lower tensile index values. FIG. 22 shows that the benefits from lowering the grinding solids below the 5% total solids (2.5% fibre solids) standard become positive when the tensile index of the MFC ground at 5% total solids is greater than about 5 Nm/g.

FIG. 23 is a graph of specific energy input (kWh/t) versus tensile index (Nm/g) for 6 different fibrous substrates comprising cellulose, including Enzyme-Treated Nordic Pine, Abaca, Blue Roll, Eucalyptus, Nordic Pine, Acacia, Birch and Cotton. Standard lab grinds were carried out with various cellulose pulp species across a wide range of energy inputs. The optimum energy input does not appear to vary much between pulp species. Differences in peak tensile index vary greatly, with Cotton being the weakest (tensile index of 4 Nm/g) and Birch being the strongest (tensile index of 12 Nm/g). Predicting the MFC tensile index properties from easily measurable properties such as hemicellulose content (mass fraction) and zero-span tensile index measurements (Nm/g) enable selection of fibrous substrates comprising cellulose to produce MFC with increased tensile index properties without the necessity of fibrillating each feed stock to determine such properties.

The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teachings can be readily applied to other types of apparatuses, experiments and surgical procedures. Also, the description of the embodiments of the present invention is intended to be illustrative and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.

It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.

References discussed in the application, which are incorporated by reference in their entirety, for their intended purpose, which is clear based upon its context.

The disclosures of each and every patent, patent application, publication, and accession number cited herein are hereby incorporated herein by reference in their entirety.

While present disclosure has been disclosed with reference to various embodiments, it is apparent that other embodiments and variations of these may be devised by others skilled in the art without departing from the true spirit and scope of the disclosure. The appended claims are intended to be construed to include all such embodiments and equivalent variations.

The foregoing written specification is considered to be sufficient to enable one skilled in the art to practice the embodiments. The foregoing description and Examples detail certain embodiments and describes the best mode contemplated by the inventors. It will be appreciated, however, that no matter how detailed the foregoing may appear in text, the embodiment may be practiced in many ways and should be construed in accordance with the appended claims and any equivalents thereof.

REFERENCES

References discussed in the application, which are incorporated by reference in their entirety, for their intended purpose, which is clear based upon its context.

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Abbreviations

MFC=Microfibrillated cellulose.

MVC=Media volume concentration

POP=Percentage of pulp.

RBA=Relative bonded area. 

What is claimed is: 1.-73. (canceled)
 74. A method for preparing an aqueous suspension comprising microfibrillated cellulose with increased tensile properties and further comprising inorganic particulate material, the method comprising the steps of: (i) providing a multiplicity of fibrous substrates comprising cellulose; (ii) determining the zero-span tensile index in Nm/g and hemicellulose content of the fibrous substrates comprising cellulose; (iii) predicting the MFC tensile index in Nm/g from the product of the hemicellulose content and fibre zero-span tensile index of the fibrous substrates comprising cellulose; (iv) selecting the fibrous substrates comprising cellulose having a desired MFC tensile index; and (v) microfibrillating the fibrous substrates comprising cellulose in an aqueous environment by grinding in the presence of a grinding medium, wherein the grinding is carried out in the presence or the absence of grindable inorganic particulate material.
 75. The method according to claim 74, wherein the grinding is carried out in the presence of grindable inorganic particulate material.
 76. The method according to claim 74, wherein the grinding is carried out in the absence of grindable inorganic particulate material.
 77. The method according to claim 74, wherein the predicted MFC tensile index is calculated using the equation T=₂ZH+σ₀, wherein Z represents the zero-span tensile index of the fibre in Nm/g, H represents the hemicellulose content (mass fraction), B2 is a proportionality coefficient, and σ₀ is a constant.
 78. The method according to claim 74, wherein the grinding medium is removed at the completion of grinding.
 79. The method according to claim 74, wherein the MFC has a fibre steepness of from about 20 to about
 50. 80. The method according to claim 74, wherein the grinding medium is present in an amount of at least about 10% by volume of the aqueous environment.
 81. The method according to claim 75, wherein the fibrous substrate to the inorganic particulate material are in a ratio of about 99.5:0.5 to about 0.5:99.5.
 82. The method according to claim 74, wherein the grinding is performed in a tower mill.
 83. The method according to claim 74, wherein the grinding is performed in a screened grinder.
 84. The method according to claim 74, wherein the screened grinder is a stirred media detritor.
 85. The method according to claim 84, wherein the screened grinder comprises one or more screens having a nominal aperture size of at least about 250 μm. cm
 86. The method according to claim 74, wherein the grinding is performed in a cascade of grinding vessels.
 87. The method according to claim 74, wherein the fibrous substrate comprising cellulose has a Canadian Standard freeness equal to or less than 450 cm³.
 88. The method according to claim 74, wherein the fibrous substrate comprising cellulose has a Canadian Standard freeness equal to or greater than 450 cm³.
 89. The method according to claim 74, wherein the grinding medium comprises particles having an average diameter in ranging from about 0.5 mm to about 6 mm.
 90. The method of claim 74, wherein the fibrous substrate comprising cellulose is present in the aqueous environment at an initial solids content of at least about 5 wt. %.
 91. The method according to claim 74, wherein the fibrous substrate comprising cellulose is present in the aqueous environment at an initial solids content of less than about 5 wt. %.
 92. The method according to claim 74, wherein the total amount of energy used in the method is less than about 2,500 kWh per tonne of dry fibre in the fibrous substrate comprising cellulose.
 93. The method according to claim 74, wherein the total amount of energy used in the method is less than about 2,000 kWh per tonne of dry fibre in the fibrous substrate comprising cellulose.
 94. The method according to claim 74, wherein the fibrous substrate comprising cellulose is selected from the group consisting of Nordic Pine, Black Spruce, Radiata Pine, Southern Pine, Enzyme-Treated Nordic Pine, Douglas Fir, Dissolving Pulp, Birch #1, Birch #2, Eucalyptus, Acacia, Mixed European Hardwood, Mixed Thai Hardwood, Tissue Dust, Cotton, Jeans, Abaca, Sisal, Bagasse, Kenaf, Miscanthus, Sorghum, Giant Reed and Flax.
 95. The method according to claim 74, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is about 15 to about 25 Nm/g.
 96. The method according to claim 74, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 15 Nm/g.
 97. The method according to claim 74, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 20 Nm/g.
 98. The method according to claim 74, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 25 Nm/g.
 99. The method according to claim 74, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 30 Nm/g.
 100. The method according to claim 74, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 35 Nm/g.
 101. The method according to claim 74, wherein the WC fibre length is about 0.1 to 0.25 mm.
 102. The method according to claim 74, wherein the WC fibre length is greater than 0.25 mm.
 103. The method according to claim 75, wherein the inorganic particulate material is an alkaline earth metal carbonate or sulphate.
 104. The method according to claim 75, wherein the inorganic particulate material is calcium carbonate.
 105. The method according to claim 104, wherein the calcium carbonate is ground calcium carbonate.
 106. The method according to claim 104, wherein the calcium carbonate is precipitated calcium carbonate.
 107. The method according to claim 75, wherein the inorganic particulate material is selected from the group consisting of an alkaline earth metal carbonate or sulphate, a hydrous kandite clay and an anhydrous (calcined) kandite clay or combinations thereof.
 108. The method according to claim 75, wherein the inorganic particulate material is selected from the group consisting of calcium carbonate, magnesium carbonate, dolomite, gypsum, kaolin, halloysite or ball clay, metakaolin, fully calcined kaolin, talc, mica, perlite, diatomaceous earth, magnesium hydroxide, and aluminium trihydrate, or combinations thereof.
 109. The method according to claim 75, wherein the inorganic particulate material has a particle size distribution in which at least about 10% by weight of the particles have an e.s.d of less than 2 μm.
 110. The method according to claim 75, wherein the inorganic particulate material has a particle size distribution in which at least about 20% by weight of the particles have an e.s.d of less than 2 μm.
 111. A method for selecting a fibrous substrate comprising cellulose for preparation of microfibrillated cellulose having increased tensile properties, the method comprising the steps of: (i) providing a multiplicity of fibrous substrates comprising cellulose; (ii) determining the zero-span tensile index in Nm/g and hemicellulose content of the fibrous substrates comprising cellulose; (iii) predicting the MFC tensile index in Nm/g from the product of the hemicellulose content and fibre zero-span tensile index of the fibrous substrates comprising cellulose; and (iv) selecting the fibrous substrates comprising cellulose having a desired MFC tensile index.
 112. The method according to claim 111, wherein the predicted MFC tensile index is calculated using the equation T=B₂AH+σ₀, wherein Z represents the zero-span tensile index of the fibre in Nm/g, H represents the hemicellulose content (mass fraction), B2 is a proportionality coefficient, and σ₀ is a constant.
 113. The method according to claim 111, wherein the MFC has a fibre steepness of from about 20 to about
 50. 114. The method according to claim 111, wherein the fibrous substrate comprising cellulose has a Canadian Standard freeness equal to or less than 450 cm³.
 115. The method according to claim 111, wherein the fibrous substrate comprising cellulose has a Canadian Standard freeness equal to or greater than 450 cm³.
 116. The method according to claim 111, wherein the fibrous substrate comprising cellulose is selected from the group consisting of Nordic Pine, Black Spruce, Radiata Pine, Southern Pine, Enzyme-Treated Nordic Pine, Douglas Fir, Dissolving Pulp, Birch #1, Birch #2, Eucalyptus, Acacia, Mixed European Hardwood, Mixed Thai Hardwood, Tissue Dust, Cotton, Jeans, Abaca, Sisal, Bagasse, Kenaf, Miscanthus, Sorghum, Giant Reed and Flax.
 117. The method according to claim 111, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is about 15 to about 25 Nm/g.
 118. The method according to claim 111, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 5 Nm/g.
 119. The method according to claim 111, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 10 Nm/g.
 120. The method according to claim 111, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 15 Nm/g.
 121. The method according to claim 111, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 20 Nm/g.
 122. The method according to claim 111, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 25 Nm/g.
 123. The method according to claim 111, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 30 Nm/g.
 124. The method according to claim 111, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 35 Nm/g.
 125. The method according to claim 111, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 40 Nm/g.
 126. The method according to claim 111, wherein the product of hemicellulose mass fraction and fibre zero-span tensile index is greater than 50 Nm/g.
 127. The method according to claim 111, wherein the hemicellulose mass fraction of the fibrous substrate comprising cellulose is between about 10% and about 25%.
 128. The method according to claim 111, wherein the hemicellulose mass fraction of the fibrous substrate comprising cellulose is 25% or more.
 129. The method according to claim 111, wherein the MFC fibre length is about 0.1 to 0.25 mm.
 130. The method according to claim 111, wherein the MFC fibre length is greater than 0.25 mm.
 131. The method according to claim 111, wherein the fibrous substrate comprising cellulose has a d₅₀ ranging from about 5 μm to about 500 μm, as measured by laser light scattering.
 132. The method according to claim 111, wherein the fibrous substrate comprising cellulose has a d₅₀ equal to or less than about 300 μm, as measured by laser light scattering.
 133. The method according to claim 111, wherein the fibrous substrate comprising cellulose has a d₅₀ equal to or less than about 200 μm, as measured by laser light scattering. 